Annette King gives Jacinda Ardern a warm embrace last August, after her first speech in parliament as the new Labour leader.

On a cool grey morning in Wellington, in the doorway of her office on the ninth floor of parliament, the 25th prime minister of New Zealand is looking about as worried as anyone with her famously enormous smile is capable of. "I'm so sorry, I've given you nectarine hand," says Jacinda Ardern, a moment after shaking hands. "I was just eating one, and you know how the juice goes everywhere. But anyhow, welcome!"

Wiggling her fingers, she leads the way into her office, a big room with a curving wall of windows.

She takes a seat at a large wooden desk and begins detangling a pair of headphones while examining a pink T-shirt with the words "Rt Hon Splorer" written on it. Adern has attended annual music festival Splore several times. "I better not strip and put this on," she says reluctantly. Did her partner, fishing show presenter Clarke Gayford, get a T-shirt, too, asks her social media editor, setting up camera gear. "Yes," says Ardern. "Something to do with fishing – I can't remember. But mine is better."

I'm not supposed to be bothering Ardern – I have permission to shadow her for two days, with an interview only at the end – and at this point, my assigned media person begins to usher me out of the room.

Ardern turns. "You can stay longer while I do my boring rattle-off thing, if you like," she smiles.

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She spends the next 10 minutes doing a series of unscripted, perfect-first-time clips for social media. Then, obviously changing her mind, she pulls her dark floral shift dress off.

She's wearing a modest black slip underneath, but still, I'm glad I'm not Charles Wooley. She puts on the pink T-shirt, then records a welcome to Splore. Smiling into the camera, she apologises that she can't be present in person, and says she's looking forward to seeing someone dressing up "in a brown wig, Labour rosette and pregnancy gut".

So, fruit juice, partial strip, self-parody. We've seen and heard a great deal about Ardern since she became prime minister last October, but clearly, there's more to the world's youngest elected head of government (until she was pipped by the new 31-year-old Austrian chancellor in December) than meets the eye.

Let's not forget that Ardern performed a political miracle last October. Amid an international climate of disastrous defeats for social democratic politics – the US, UK, France, and Italy have all rejected their centre-left parties in the past 18 months alone – this 37-year-old woman led the Labour party to victory after almost a decade in the political wilderness, having taken over the leadership less than eight weeks earlier.

Ardern had been an MP for nine years but had no ministerial experience. Yet after a 54-day campaign – "Let's Do This" – she did it. She got Labour to within 10 seats of the National party, which had been in power for three three-year terms (almost all of them under John Key). Then she conducted weeks of coalition negotiations that – against all expectations – snatched electoral victory.

"She was meeting with the Greens in one room, and New Zealand First in the other, and she kept both partners in the tent, talking," recalls Annette King, former Labour deputy leader and 30-year political veteran, who was on the negotiating team. "And don't forget, they were talking to the National Party as well. She was moving from room to room, morning and afternoon, day after day. She had to hold in her mind exactly what she was negotiating – you can't make a promise to one party and then renege on it with the other. It was a massive feat, and she led it all."

Along with negotiating skill, Ardern had charisma on her side: she's one of those intensely likeable people that almost everybody, well, likes. As David Farrar, a right-wing pollster, blogger and ex-National Party staffer (so theoretically not a Jacinda fan) puts it: "Jacinda herself is very warm, very genuine, very comfortable in her own skin."

Certainly, every time I go somewhere with Ardern – whom the entire country seems to call by her first name – there's a feeling that can only be described as giddy. Grown men and women smile and laugh when they see her; they rush up for selfies; they clutch their hearts with excitement; they hug her – often – and hold her hand. At the first event I attend, the opening of a building at Victoria University in Wellington, someone gifts her a grey onesie for the baby she's having in June. Someone else helps her hold a tuatara. "He may bite," I can see the worried handler mouthing. Predictably, the lizard relaxes in Ardern's hands, legs dangling.

Jacinda Kate Laurell Ardern was born on July 26, 1980. She has one older sister, Louise, and just before she started school, her family moved to the Bay of Plenty town of Murupara, infamous during the 1980s for gang violence. Her father Ross was the local police sergeant, her mother Laurell worked in the school canteen.

It sounds like a dramatic place to be a little kid: their house was pelted with bottles, the man who lived next door committed suicide , and the family babysitter turned yellow from hepatitis C. Ardern once recalled sneaking barefoot out through the back fence and coming upon her dad being confronted by several scary-looking men. "Keep walking, Jacinda. Keep walking," he told her.

Reading between the lines, one suspects she was one of those super-bright, super-nice kids you sometimes come across: a star debater, a school defender of the vulnerable (aged 5, she stood up to kids bullying her older sister), and the underprivileged – those who had "no lunch and no shoes".

HENRY COOKE/STUFF Jacinda Ardern tours a tent school in Tonga earlier this month.

At home: "I was always trying to fix everything," she explains. "I was the peacemaker. I remember hearing my sister packing her bags once to run away, and slipping a note under her door begging her not to go. I would have been so irritating!"

She studied politics and communications at Waikato University, and was employed as a staffer in Helen Clark's government in the mid-2000s before a stint in London, where she headed the International Union of Socialist Youth and worked for Tony Blair's Labour machine. When she returned, she entered politics in 2008 as a list MP.

She was 27, the youngest sitting member of parliament. One of her former teachers, Gregor Fountain – whom she invited to her swearing-in as PM – recalled her "amazing ability and curiosity. I remember her staying behind in class to talk about issues, because she really wanted to grapple with them." At the end of high school, her year book contained various "Who's most likely…" descriptions. Ardern's category? "Most likely to be prime minister."

Her mother and father were major influences and the family are close, though her scientist sister lives in London and her parents live in Niue, where Ross Ardern has been High Commissioner since 2014.

Ardern was raised a Mormon: her nanna was converted via the classic Mormon doorknock and the rest of the family followed. And even though she left the church many years ago (mostly over its rejection of homosexuality), there's no breach with her family.

"I can't separate out who I am from the things that I was raised with," says Ardern. "I took a departure from the theology, but otherwise I have only positive things to say about it." She's retained certain Mormon characteristics: the positivity, the surprising openness, the at times almost painful sincerity.

"I'm really earnest," she agrees. "I think it annoys people! I asked a reporter about it once." She laughs. "And she said, 'Um, look, yes, maybe.'"

But if she's earnest, she's also ballsy: and perhaps that's a Mormon legacy, too. "I've never had any hesitancy in talking to people," she says. "If I've got a purpose and I need to go and speak to people, or knock on doors, I will. I don't mind doorknocking for politics." She grins. "Because nothing is as hard as doorknocking for God!"

The morning after we meet, I travel to Christchurch with Ardern's entourage. Or rather, in front of it. Ardern, who uses commercial domestic flights, boards last, at the very back of the plane, and I'm not even sure she's present until I see her after we land: a tall, black-clad figure striding across the tarmac pulling her wheelie bag, nodding (earnestly) as an elated-looking air steward talks beside her.

Her first meeting is to commit $10 million to the restoration of earthquake-damaged Christ Church Cathedral. Various officials stand around awkwardly until she arrives and gathers them up with a big embracing motion. "This isn't staged at all, is it?" she jokes cheerfully, as everyone shuffles towards the cameras. She gives a little speech, explaining that "the people behind me here have actually done all of the work".

She performs this praise-deflecting manoeuvre repeatedly.

At Canterbury University later in the day, when an official recalls that she was "mobbed" by students during her last visit, she immediately corrects him. "I don't think I was mobbed," she says with a smile. "It was raining, and I had an umbrella." The next day, when US Vogue publishes a profile piece including a photograph of her looking like a supermodel, she responds by posting a picture on social media of her as a little kid with a "full '80s mullet".

It's as if it's impossible for her to take a compliment, I say. "That's actually true," admits Ardern, sounding surprised. "No one's ever said that to me before."

After the cathedral, Ardern's entourage heads 500 metres down the road for the launch of an electric car share company.

Off to one side, a small knot of protesters have gathered holding placards about their seven-year fight over earthquake insurance claims. Megan Woods, the local MP, wades in.

After a minute, I realise that Ardern, last seen being ushered towards the red carpet, is standing beside her. The protesters seem surprised. Someone called Gary begins to ask a very long question about obstruction and incompetence and the loss of his life savings. "OK, Gary, that's enough," says someone else, and Gary subsides. Ardern, who has been listening and nodding, says: "What's the best way for us to communicate with you?" Several big trucks roar past, and she pauses. "Because there's a long list of stuff we're doing, and we want to make sure you hear about it every step of the way." She shakes hands with several protesters, who look thrilled, then sets off back to the launch, holding a bag containing another onesie – white, this time.

Ardern's next function is with families of the 115 people who died when the Christchurch Television (CTV) building collapsed during the earthquake.

GETTY Barack Obama poses with New Zealand Prime Minister Jacinda Ardern at Government House on March 22. It was the former US president's first visit to New Zealand.

Late last year, after six years, four investigations and millions of dollars, New Zealand Police announced it would not prosecute the building's engineers for negligent manslaughter due to inconclusive and contradictory evidence.

The meeting takes place in a church and is closed to journalists, so a big group of us wait in a stiflingly hot anteroom. Someone is playing an organ at high volume close by, a long dirge which seems appropriate to Ardern's words when she emerges.

"I felt not only a duty of care to come and speak to the families today," she says, "but also just from a human perspective I felt it was important, being in the position I'm in, that we do everything in our power to prevent such a tragedy from happening in the future."

She goes on to explain that many of the families are taking comfort from the idea of future legislation to allow for prosecution in similar scenarios. But the families' representative – Professor Maans Alkaisi, who lost his wife, GP Dr Maysoon Abbas, in the collapse – looks entirely uncomforted when he appears. He wants a judicial review into the police decision and money from the government to support the families in a civil case.

"[Ardern] is a wonderful person," he says, "very sincere. But she is considered to be one of the most influential female politicians in the world. We feel that if she asks for something, she can get it."

The CTV case, which one commentator describes to me as "a deep, painful open wound", has the potential to become a problem for Ardern, as does the complex rebuilding of Christchurch as a whole. She campaigned on empathy and fairness, but empathy, however sincere, does not heal all wounds. If it leads to false hope, in fact, it can actually make things worse.

During parliamentary sitting weeks, Ardern typically spends Monday to Wednesday in Wellington, and Thursday visiting a regional area. On Fridays, she heads home to Auckland. The day following her return from Christchurch, I arrive at her home for our interview.

Ardern currently lives in Point Chevalier in Auckland (though a few days after we meet it is announced she and Gayford have bought a four-bedroom home 10 minutes down the road in Sandringham).

This is a startlingly normal-looking suburb, and her house is a modest, single-storey brick and tile building. It's indistinguishable from others on the street; except, that is, for the two security guards, one old and one young, who travel everywhere with her.

They are standing against a dark fence that looks mostly like a fence you'd buy at Bunnings, and slightly like a fence that could repel a tank attack.

Ardern is inside, wearing another shift dress and tights, but no shoes. She pours me a lemonade ("homemade, but not by me") and sits on her low grey couch in her bright living room, opposite a TV and a large heap of shoes notionally piled into a basket. Outside, there's a little patch of grass, some hopeful jasmine and lots of small weeds beside the window – the garden of someone who's rarely home.

You must be exhausted, I say: all those events, all that hand-holding and hugging. "I like it!" she exclaims. "Partly it's probably me: I always put my arm around them.

During the election campaign, people would say every day, 'Can I hug you?' and I'd say, 'Of course you can!' I think it's wonderful if people think I'm accessible enough that I can do that – I take it only as a compliment."

She pauses for a moment. "Of course, I do get angry, and upset," she says. "Not with the hugging – but I am a normal human."

This is slightly surprising: one of Ardern's favourite words is "robust", and she often seems to brush off political criticism – not to mention obsessive interest in her personal life – with phrases like "it was a robust debate" or "I'm pretty robust". So what's her technique for managing stress? "Well, if something's bothering me, in order to really work it through, I talk it out a lot. That's my way of processing stuff." She smiles. "Sometimes that means people around me have to put up with a lot of chat."

These things, she goes on, are part and parcel of political life: a life she chose a long time ago. She was handing out Labour leaflets at 17; three years earlier she interviewed Marilyn Waring – former National Party MP, now a noted feminist and academic – for a school project.

"I thought her courage was phenomenal," Ardern recalls. "So I went down to the school canteen where my mum worked, and found her phone number. And of course she didn't pick up, and I left this long garbled message, as only a 14-year-old could do."

A few weeks later, Waring called her back. "What I really remember about Jacinda was that she had specific issues she wanted me to address," recalls Waring, now a professor of public policy at the Auckland University of Technology. "'What do you think are the key issues facing my generation? What do you think about a nuclear-free New Zealand?'"

Today, Waring feels hopeful about Ardern's election, and also very relieved. "I can't tell you: my generation [Waring was born in 1952] has cruelly stuffed it up – whether it's the free market bullshit, the environmental devastation, or the incredible gap between rich and poor.

"Political transitions have to be dynamic like this – maybe like Canada, too – because if you've just got the old boys hanging on, they make sure you never get to prove that they did anything wrong, and so you can never get anywhere."

Now that this country has chosen Ardern, "let's just hope she can keep being herself, and having this lovely candour, and enabling us to trust. Politics is a terrible trap: you can't transform it all on your own because the structures all work in such patriarchal Victorian ways."

Even for those such as Waring, who have faith in Ardern personally, this is a worrying issue. As pollster David Farrar says: "Labour has so few really good ministers, they've had to load all the really important stuff on to just a few people, and they're struggling."

The composition of Ardern's government won't help. Any coalition is inherently unstable, points out Farrar: the more actors, the greater the potential for disunity. "The history of small parties in government in New Zealand is that they've all lost votes in the next election," he explains.

"The problem is that if you, as a voter, think the government is doing a good job, why wouldn't you choose Labour next time?"

This is, theoretically, good for Ardern, but small parties can become disruptive and hostile in a bid to avoid destruction, which can make government difficult. In this case, head of New Zealand First and Deputy Prime Minister Winston Peters has fallen out with just about everyone, in every party, during his long political career. And though the Greens co-leader James Shaw is a personal friend of Ardern's, his party's vote has already dropped to 5 per cent – the cut-off for presence in parliament. "In 18 months," says Farrar, "the pressure could really be on."

Speaking of pressure, by June, Ardern will not only be PM, but also a mother. She discovered she was pregnant soon after coalition negotiations; and as is her way, she's already announced lots of plans about what's going to happen and when. She'll take six weeks of maternity leave, during which Winston Peters will be acting PM, then she'll return full-time, and Gayford will become the primary carer.

DAVID WHITE/STUFF Former PM Helen Clarke embraces Ardern during the Labour Party campaign launch last August – one of many hugs on the trail. Does all the hand-holding and hugging become a chore? “I like it!” she exclaims.

Gayford and Ardern met four years ago, when he (belying his knockabout radio DJ-cum-TV fisherman image) contacted her with his concerns about privacy legislation.

In his retelling, they met for a coffee and he discovered, to his amazement, that she liked Concord Dawn – "a fantastically awesome heavy drum'n'bass outfit". He took her fishing, she caught a 5½-kilogram snapper, dolphins and whales frolicked on cue. The rest, as they say, is history.

Gayford has previously described his main role as the PM's partner as, "Just to make sure that she's OK and be in the background going, 'Have you eaten your lunch? Have you slept properly? You've got lipstick on your teeth.'"

Perhaps unsurprisingly, he seems unfazed about fatherhood. "It's not like we haven't been through a few changes in the past year," he points out genially, speaking on the phone from Wellington. "Although I'm lucky, because I get to dip in and out of her world. I still get to bugger off and go fishing, where everything's exactly the same. But then I come home and put on a suit and go to an awards dinner where they announce your name as you come into the room."

What has her being PM done to their home life? "Well, I have been making jokes recently that there's three of us in the relationship now," he laughs. "Me, her and the cabinet papers. And the cabinet papers appear just in time to ruin every weekend. They come in this big, security-coded briefcase, and it's my job to go out and get it from the gate on Friday. And I gauge the severity of my weekend based on the weight of that bag.

So I walk in, and I stand there, and she looks at me and goes, 'OK, so what do you think?' And then I go, 'Not a good weekend, darling.'"

Early parenthood doesn't lend itself to good weekends either, of course, but one of the unexpected details about Ardern's impending motherhood is that, in fact, her work scenario is surprisingly baby friendly – at least on parliamentary sitting days.

"We've got several newly elected members of parliament who have babies," explains Annette King. "And the Speaker of the House, Trevor Mallard, is called the Baby Whisperer: whenever there's a baby around, Trevor's got it."

It is not unusual to see Mallard, in the Speaker's chair, holding a baby while overseeing debates (he also apparently has a cot in his office), or members breastfeeding in chamber.

Ardern explains that Gayford will bring the baby and travel with her if need be, and that they're open to "friends and family" helping out: basically, her plan seems to be to keep doing the job as she does it now. "I don't have many choices work-wise," she explains. "No one's saying, 'How would you like your work and home arrangements to be?' It just is what it is. So there's no guilt, because if I want to do this job, there's no choice."

For all the initial reluctance, and the less than ideal preparation and timing, there's no doubt Ardern does want to do this job. "I do enjoy it enormously," she says, almost sheepishly, tucking her legs underneath her. "It's a job about spending time with people, advocating on their behalf, and making decisions for New Zealand. That's what drove me into politics in the first place."

A few days after our interview, Ardern makes a lightning trip to Australia, where I see her at a business lunch with PM Malcolm Turnbull. An unusually ebullient Turnbull starts his speech by telling everyone that Ardern came to his house the previous night for dinner, like a schoolkid boasting that the popular girl went to his party.

Ardern, in turn, is enthusiastic and charming.

She avoids (on this occasion) mention of tensions over university fees and criminal deportations, and makes much of Australia's position as New Zealand's valued trading partner (second only to China), while saying the historic trans-Tasman bond is as strong as ever. She also mentions a new domestic policy: a standard-of-living framework she plans to build in to the 2019 budget, measuring not only economic but social success.

"Yes, balancing the books matters," is how she puts it to me. "But so does making sure that your people aren't sleeping in cars, and your children aren't living in poverty."

She wants environmental and social measures, as well as economic ones, put in place so that we "can understand where our investment and spend is going and the impact of what we're doing".

If she pulls it off, Ardern's will be the first government in the OECD to implement "wellbeing economics" in a meaningful way.

Who knows if she'll achieve this scheme – or any other, for that matter. Her story, so far, reads like a political fairytale, but other wildly popular leaders – Tony Blair, Barack Obama – all lost support as the realities of power set in.

However beloved you are, once you are faced with tough decisions, which have winners and losers, you will inevitably disappoint people. Canada's Justin Trudeau, another charismatic, liberal leader, saw his disapproval rating jump above 50 per cent this month for the first time since his 2015 election. If things don't work out in this bold political experiment, it won't be Labour or the government that takes the blame – it will be Jacinda Ardern.

Time will tell whether she has the political intelligence, endurance and luck to navigate this; if she has the ability to lead the nation safely through the shoal waters of 21st century politics.

Still, in a world in which we're increasingly expected to accept alternative facts, and indefinite strongman rule, and threatening, isolationist policies from world leaders, it's nice to be offered something – and someone – different to believe in. As Ardern puts it, barefoot in her modest house: "I don't think too much about the magnitude of the job.

I just immediately skip to, 'Let's get the plan going.'"