Her single Royals has sold 1m copies in the US, has knocked Miley Cyrus off the top of the charts and her much-awaited debut album Pure Heroine is about to be released. But 16-year-old New Zealander Ella Yelich-O'Connor is unfazed by impending global stardom

A middle-aged woman walks into the lobby of a west London hotel, stops and shoots a quizzical glance in the direction of the table where Ella Yelich-O'Connor and I are sitting. Yelich-O'Connor interrupts her explanation of why she chose Lorde as her stage name – "I wanted an aristocratic title, but I wanted it to look feminine, like, aesthetically" – looks up and waves her away – "We're just doing an interview here" – before turning back to me. I assume it's an American tourist who has recognised her. When I meet Lorde, her debut single Royals has just sold 1m copies in the US, broken Alanis Morissette's record for the longest reign by a female artist at the top of Billboard's Alternative Chart and will – in a couple of weeks' time – dethrone Miley Cyrus from the top of the Hot 100. But I assume wrongly. "No, that's my mum," she sighs.

Interviewing an international pop sensation while their mum hovers in the background is a slightly odd experience, but then, as Yelich-O'Connor points out, she shouldn't really be here herself. She should be back home in New Zealand, in Davenport, a suburb of Auckland that's apparently known locally as the Bubble – "because it's so insular and closed off from everything" – and which she describes, winningly, as "the kind of suburb that people make movies about, there's quite weird mums everywhere".

"I haven't quit school," she frowns. "Technically, I'm still there. I don't really know how it's going to work. I'm a realist about the effect that doing what I'm doing now is going to have. You know, it's enough of a full-time thing. I actually feel like I'm doing work as opposed to sitting around waiting for my life to start, which is pretty awesome." Her thoughts about her education are interrupted by the arrival of her lunch. "Olives!" she exclaims delightedly. "I love olives. I'm not allowed to have any alcohol backstage, so I always ask for olives."

The chaperoning mum, the concerns about school and the alcohol-free rider are all reminders that Yelich-O'Connor only turned 16 last year: useful, because talking to her, you could easily forget. On the one hand, she talks like a teenager – there's a lot of likes and you knows and awesomes and rads – but nevertheless, for a 16-year-old who had never left New Zealand before this year, and who currently finds herself in the middle of a vertiginous rise to stardom, she appears almost weirdly unfazed and self-possessed. She seems to view what's happening to her – the record sales, the gushing celebrity endorsements (everyone from the Weeknd to Elton John), the "wave of shit" she's provoked by offering some fairly negative opinions in interviews about Justin Bieber and Lana del Rey – with a kind of cool, wryly amused detachment. By the time she arrived in the US, she notes, Royals was so big that fans turned up at the airport and outside her hotel. "Which was weird," she says, with a perplexed shake of the head. "People at the airport. 'You're, like, really cool!'"

Perhaps her calmness has something to do with New Zealand, a country that, by her account, doesn't really go in for excitable celebrity culture: "I've had two of the biggest songs in the country and I can do exactly what I've always done. I can walk around, go to parties with my friends, it's still relatively casual." Or perhaps it's the result of what, by anyone's standards, seems a pretty unique musical apprenticeship. The daughter of a civil engineer (dad) and a writer (mum), she was signed aged 12 – "so young I didn't really feel like it was that much of a big deal" – after a record label talent scout saw a video of her singing in a school concert, "doing Warwick Avenue by Duffy. Not cool. Sorry."

They suggested she put out an album of soul covers: she refused, telling them she wanted to write her own material, even though she had never written a song in her life, only short stories (albeit short stories influenced by Raymond Carver, her favourite author "when I was a kid") and had no idea what she wanted her music to sound like. "They were pretty open-minded about it," she shrugs, as if a major record label taking orders from a 12-year-old who doesn't seem to have a clue what she's doing is the kind of thing that happens all the time. "They got straight away that I was a bit weird, that I would not be doing anything I didn't want to do, and they completely went with that. I dunno," she says, noticing my faintly incredulous expression, "maybe it was because the record company was in New Zealand. Anyway, it's cool that they were cool with that, because if they hadn't been," she laughs, "it wouldn't have been a very good outcome."

Lorde performs at the Decibel Festival, Seattle. Photograph: Mat Hayward

As it turned out, it took her a couple of years, and the arrival of co-writer and producer Joel Little, to come up with the songs she began posting online last year, insisting that her record label "leave it alone – don't promote it, no ads, let it grow organically" (things began moving when singer-songwriter Grimes tweeted a link to Lorde's Soundcloud page, having being alerted to its existence by "some random"). Some of the songs have turned up on her debut album Pure Heroine. You can understand why people have compared its chilly, hip-hop influenced electronic pop to Lana del Rey, but the real difference lies in the words, which Yelich-O'Connor writes alone, and which are uniformly fantastic, big on withering critiques of the glaring disparity between the moneyed glamorous lifestyle pop culture presents to its teenage consumers and the reality of their suburban lives.

She seems a little surprised by the amount of attention the lyrics have received, protesting that the point she's making – that pop's recent obsession with champagne-fuelled VIP-room high life has "nothing to do with anything that's happening in Auckland" – is a really obvious one. "Writing Royals was sort of like: 'Why the fuck isn't anyone talking about this?' I feel like I'm late to the party saying it, but actually I'm not, there's nothing here, how is there nothing here? And so all these people started telling me that what I'm saying is, like, profound. Are you serious? No, it's not. It's hugely concerning for me."

Not, she quickly points out, that she and her friends ever hankered after that kind of lifestyle: "We go to quite, like, grotty parties with super-weird punks, our lives aren't super-aspirational to the pop world or anything." But she still thinks it has a kind of trickledown effect. "Everyone is so obsessed with how everything looks, how the party will look through a lens the next day. We all have Tumblr and we all have Instagram and everything. People care so much about it because, now, any random can be famous on the internet if their world looks good on Tumblr. And so everyone at high school strives for this kind of aesthetic correctness. I do it as well, you know. I curate my life in a way. It's always playing on my mind, kind of a love-hate relationship. I'm not one of those people who's, like, 'I wish Facebook wasn't around,' because, you know, it is what it is."

She sighs. "I'm like the most terrible person to go to a party with in the world, because I just can't enjoy it. I'm just thinking all the time about what it means and what the implications are. So everybody's getting fucked up and I just can't give myself over to it, because I'm thinking about" – she lets out a self-mocking laugh – "the archetypes of being a teen. I'm really interested in kind of weird social situations and cliques, watching girls vying for attention, watching how the popularity thing happens. I've always thought way too hard about everything." Another self-mocking laugh. "Not normal. It's really not healthy. I'm just a freak," she says, before turning back to her olives.

Pure Heroine is released in the UK on 28 October on Virgin Records.