New Australian Greens leader Richard Di Natale wants to find common ground with the coalition, but has vowed to fight for health, education and climate action.
The Victorian senator, who worked as a GP before entering parliament, was on Wednesday unanimously elected leader in a party room meeting after Christine Milne's retirement announcement.
In an unusual step, deputy leader Adam Bandt was replaced by a team of two deputies, Queensland senator Larissa Waters and WA's Scott Ludlam.
The party declined to give any reasons why Mr Bandt was dropped as deputy, or confirm speculation Senator Di Natale had been tipped off early about the leader's resignation.
However, he said he had spoken to his wife Lucy six months ago about the prospect of becoming leader should Senator Milne retire.
Senator Milne, who won't stand for her Tasmanian senate seat at the 2016 election and will step down before her term expires in mid-2017, said she wanted to spend more time with her family and allow generational change in the party.
Senator Di Natale's immediate challenge will be to respond to the federal budget on May 12, and plans to meet Prime Minister Tony Abbott next week.
"I'm going to talk to him and see if there's areas where there's common ground," Senator Di Natale told reporters.
"Sadly, I don't think there are many."
Treasurer Joe Hockey wants the Greens to reconsider their opposition to the restoration of fuel tax indexation, which he says is in line with the minor party's environmental agenda.
Senator Di Natale said despite it being a "deeply ideological" government there could be room for discussion around issues of fossil fuel subsidies, pensions, health care, education and climate.
Tough action on climate change was a moral, social and economic responsibility, he said.
The son of Italian immigrants said he would champion multiculturalism and take up the Greens' battle to end offshore processing of refugees and get children out of detention.
"We are the natural home of progressive, mainstream Australian voters," he said.
"We're going to give voice to their concerns, decent health care, decent education, affordable housing, public transport."
Labor frontbencher Chris Bowen said the Greens had engaged in a "closed door, backroom deal" to elect a new leader, in contrast to Labor's caucus and grass-roots member ballot for Bill Shorten.
Senator Di Natale is only the third leader of the Australian Greens in the party's history and the first to come from the mainland after two Tasmanians, Senator Milne and Bob Brown.
The Greens have 10 senators and one lower house MP.
Opinion polls put its primary vote near 12 per cent nationally, with support strongest in inner-metropolitan areas.
