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Just over a week ago, Christine Milne’s office was briefing that she was poised for a dramatic tactical shift that could lead to the Greens dealing with the government on controversial measures.

It was only on Wednesday, when she resigned as leader of the Australian Greens, that the extent of that tactical shift became clear, with her successor Richard Di Natale positioning himself as a “mainstream progressive” who was open to talking to both major parties and intent on poaching their voters.

Until now, Milne’s time as leader had been characterised by the party’s staunch opposition to nearly everything the Abbott government proposed. But there were signs she was preparing to move away from this approach, which had been derided by her critics as being so obstructionist as to make the Greens irrelevant.

The swift changeover was portrayed as bloodless but left some in the party room muttering that it was a stitch-up and an ambush.

She had publicly signalled a willingness to talk to the government about reducing the part-pensions of wealthy retirees. She had also written to Prime Minister Tony Abbott on March 17, outlining her party’s proposals for raising revenue.

“In addition, there are several measures that could be legislated to crack down on tax evasion and offshore tax havens which would raise billions,” she wrote to Abbott, in a letter which lashed out at last year’s budget but also reached for a conciliatory tone. “I would be pleased to provide you with further details on the above measures,” she signed off, perhaps optimistically.

The message being sent by Milne’s office was a reversal from her post-budget declaration last year that, as she felt she could not trust Abbott, she saw no point in meeting with him.

Last week, Milne herself was not available for interview to confirm the alleged shift. To this reporter, it seemed at that stage like the germ of something interesting, but not yet strong enough for a story, if Milne would not go on the record. Exactly a week before she resigned, a source close to Milne said that, given the government was struggling and under pressure to deliver a successful budget, “Christine sees an opportunity to raise some revenue from places Abbott normally wouldn’t touch and so she has changed tactics.”

“If they are genuinely willing to shift the burden onto the rich and away from people who are struggling, then let’s talk,” the source said, agreeing to be quoted though not named. “And if that’s not genuine, we’ll tell them to go jump.”

1 . Preparing for the changeover

In light of her surprise resignation as leader on Wednesday, it seems that even that source, in Milne’s inner circle, did not realise that tactical shift would involve her relinquishing the leadership.

By starting to reposition herself, Milne could have been preparing for the changeover or perhaps taking a final shot at a different leadership style as speculation mounted in the party over whether she would stay on.

After 25 years in state and federal politics, the last three as leader of the Australian Greens, Milne told her colleagues at a meeting in Canberra about 10am on Wednesday that she would not recontest the next election.

Pledging to continue to fight global warming, Milne said she felt the party, with 10 senators and one MP, was in a good position and “ready to fly”. The former teacher and avid gardener, who turns 62 next week, said she looked forward to more time with family and friends and to the arrival of her first grandchild. She has not ruled out writing a book.

It is doubtful whether Milne could have achieved a rapprochement with Abbott and the Coalition, so deep is the antipathy and distrust between them. Still, the prime minister was gracious in his tribute to her this week, saying he respected her “commitment” and they had always had “good and cordial relations”. She had held face-to-face talks with Abbott only twice since he became prime minister, coincidentally on the same date of 2013 and 2014. She left each October 22 meeting disappointed.

2 . The successor

Her successor, Di Natale, may have a better chance of executing the tactical shift that Milne was apparently entertaining. In his first comments as leader, the Victorian senator declared he was interested in outcomes, not ideology, and would be prepared to negotiate to get them.

“I’m prepared to work across party lines. I’m in there to get outcomes. I want to get things done,” he said. “I’m not an ideologue, I’m not going to come in here and say we want smaller or bigger government. We want government that looks after people. People want access to healthcare, to education and they want the environment looked after. They want clean air and water for their kids, pretty basic stuff.”

He would seek a meeting with Abbott, he said, to see if there were any areas where they could find common ground. “Sadly,” he added, “I don’t think there are many.”

Although Di Natale is passionate about the environment, he differs from the Greens first two leaders, Bob Brown and Milne, in that he came to the party from a public health background rather than as a tree-hugging crusader.

He is also the first mainlander to lead a party that grew out of the Tasmanian environmental movement.

Di Natale’s achievements since joining the senate in 2011 include securing $5 billion from Labor for Medicare-funded dentistry and forcing the Future Fund to divest $250 million in tobacco stocks.

Labor senator Doug Cameron, who has served with Di Natale on a parliamentary committee on health issues, describes him as “hard-working, intelligent and effective at what he does”. But in a precursor to the battles that loom between Labor and the Greens, Cameron added: “If Richard wants to be a mainstream progressive, he should join the Labor party.”

Di Natale’s ongoing campaigns include working with Liberal and Labor MPs to make cannabis legally available for medicinal purposes and pursuing euthanasia law reform through his Dying with Dignity bill.

The 44-year-old GP has worked in Aboriginal health in the Northern Territory, with drug users in India and travelled to Africa last December to investigate the ebola response. A self-confessed sports tragic, he comes from a Labor-voting family of Italian heritage.

Di Natale was elected unopposed within hours of Milne announcing on Wednesday morning that she would resign. Having summoned colleagues to Canberra for a pre-budget planning session, Milne told them at a morning meeting that she would step down. The MPs were then given just over an hour before reconvening to decide the leadership.

3 . Party room mutterings

The swift changeover was portrayed as bloodless but left some in the party room muttering that it was a stitch-up and an ambush, with some senators assumed to have been given advance warning, while other potential candidates were given little time to canvass support.

Within the party there had been speculation for some months about whether Milne would recontest her senate spot at the next election. Nevertheless, it caught some of her parliamentary colleagues off-guard.

Although Brown professed to be surprised by the announcement on Wednesday, Milne had discussed with him around Christmas the possibility she might step down.

NSW senator Lee Rhiannon voiced frustration at the rapid transition, saying she believed that in the interests of transparency, the party’s grassroots members should be given a say in electing a leader. That afternoon, she told Sky News the process was too quick, that she “personally would have liked more time” and that the party had “learned some lessons today”.

“We need transparent processes,” she told The Saturday Paper on Thursday. “Otherwise we could run the risk of people concluding, fairly or otherwise, that some MPs have got together to engineer the outcomes that they want.”

A review of the Greens constitution last year rejected a grassroots vote on leadership, partly because the protracted leadership wrangles of the Australian Democrats had contributed to the party’s demise.

West Australian senator Scott Ludlam who, along with Queenslander Larissa Waters, was elected as co-deputy leader, said Rhiannon’s complaint that the process was too quick was disingenuous.

“People had the opportunity to express any concerns at the time and they didn’t, so for them to express it afterwards is really surprising,” he told The Saturday Paper.

Walking into the 11.30am party room meeting, Tasmanian senator Peter Whish-Wilson said he was “shell-shocked”.

The snap leadership switch had elements in common with Milne’s ascent to replace Brown, which was only publicly announced after the fact. This time, though, there were several possible contenders for the role and no obvious, anointed successor.

Brown himself, in a radio interview after Milne announced her resignation but before the party room vote, mentioned her deputy Adam Bandt and Whish-Wilson, a former investment banker, as possible leaders. Yet Bandt, the party’s only lower house MP, was dropped from the leadership line-up.

Ludlam’s strong showing at the WA senate election rerun last April had led some to speculate he could be the next leader. As to why he did not seek the top job, he says: “I think Richard is a better bet, to be honest”.

He devised the job-sharing proposal. Waters is an articulate environmental lawyer whose promotion is designed to boost the profile of the party in Queensland. It was also, according to party sources, considered important to include a woman in the leadership team.

4 . Strained relations

Di Natale was dismissive of colleagues’ frustration at the process. “Someone may have been disappointed with the outcome,” he said, under repeated questioning in his first press conference. “Surprise, surprise. That’s politics, right?”

Bandt refrained from commenting beyond this tweet: “Congrats Richard & new team! V happy to hand over Deputy to focus on new baby (due in few wks!) & winning further Reps seats in Vic & NSW.”

Party insiders say Bandt, a former industrial lawyer, had a strained relationship with Milne, as did South Australian senator Sarah Hanson-Young, who had in 2010 unsuccessfully challenged Milne for the deputy leadership.

By the time the leadership was officially in play, Di Natale had already locked in enough support to prevent a showdown with Bandt, with at least five solid votes behind the senator. By avoiding a vote, it allowed the party to claim unanimous support for the new leadership team.

5 . Keeping a secret

When parliament last sat, on Thursday March 26, Milne was keeping a secret from her colleagues. She knew it would be her last parliamentary day as Greens leader. And it was a good one. For one thing, Peter Greste, whose cause she had championed long before it was fashionable, was freed from an Egyptian jail and celebrating in Canberra. It was a bittersweet moment for Milne, savouring the day but not taking colleagues into her confidence at that stage.

She had already decided a couple of months ago when she booked a spot at the National Press Club to deliver a pre-budget speech on Thursday that it would be her valedictory, her last chance to decry the “neo-Liberal agenda” of the Abbott government.

With the possibility of a double dissolution election looming this year and senate preselections opening in Tasmania, she knew she had to decide sooner rather than later whether to commit to another six years. So, as her office prepared for a tactical shift, she was preparing for her exit.

A politician of conviction and passion whose tough exterior belies a personal warmth, Milne had a challenging gig taking over from Brown, who enjoys a cult following that reaches even beyond the Greens’ membership.

“She decided not to step into Bob’s shoes but to wear her own,” says Ludlam. “She has taught me a lot about heart in politics and come out of it with dignity at a time of her choosing.”

The most trenchant criticism of her leadership came from former Greens staffers, now at The Australia Institute. Executive director Richard Denniss last year accused her of making the Greens irrelevant by pursuing a “strategy of voting against virtually everything the Abbott government announces, including things they actually support”.

Milne did try early to deal with the Coalition government, supporting Joe Hockey to lift the debt ceiling in December 2013, though she felt he did not keep his side of the bargain, to include a dedicated section on the environment and climate change in the Intergenerational Report.

On Thursday, she sounded a warning to her colleagues that it would be difficult to do deals with a government that could not be trusted.

The toughest criticism from Denniss was of the Greens refusal to support an increase in the petrol tax, announced in last year’s budget. This measure must now be legislated by September and Di Natale has signalled he is open to discussing it with the government, though he has also repeated Milne’s argument that the funds should be dedicated to pubic transport.

Under Milne’s leadership, the party shed some 500,000 primary votes in the senate, but grew its parliamentary representation to an all-time high. The lower house vote dropped from 11.76 per cent in 2010 to 8.6 per cent in 2013.

Di Natale says he sees no reason why the party’s vote, which consistently polls around 12 per cent, should not rise to 20 per cent as it broadens its appeal to mainstream voters, chasing seats in both upper and lower houses.

In a sign that he intends to go after the voter base of the two main parties, his first speech quoted writer Guy Pearse’s observation that: “The Greens are more Labor than Labor, more Liberal than the Liberals and not surprisingly far Greener than both.”

As the Greens’ spread from the isle of their birth to the mainland, this new “mainstream” leader is coming after both major parties. It remains to be seen if he will cut deals with them or just try to steal their votes.