The Greens will elect a new leader on Wednesday morning after Christine Milne revealed she would not contest the 2016 election and resigned as party leader.

The announcement, which was kept under wraps by the Greens leader's office, was made by Senator Milne via her Twitter account on Wednesday morning.

"Feeling optimistic, proud and sad that to announce I'm not contesting the 2016 election, and so I resign as Leader of Australian Greens," she wrote.

In a statement, the party said Senator Milne had informed Greens MPs and senators on Wednesday morning and a ballot to elect a replacement would take place at 11.30am.

Greens senator Richard Di Natale tweeted on Wednesday that he would stand as a candidate for leader, and Sky News reported that current deputy Adam Bandt and Senator Scott Ludlam are also nominating.

The preselection process in Senator Milne's home state of Tasmania for the 2016 election opens shortly.

Senator Milne was elected to the Senate in 2004, was named deputy leader in 2008 and became leader in 2012 after the resignation of Bob Brown.

She entered politics 25 years ago when she was elected to the Tasmanian parliament and became the first woman to lead a political party in Tasmania in 1993.

Senator Milne said she had made the decision to resign after speaking with her family.

"It is with a mix of optimism, pride, excitement and sadness that I am resigning the leadership and leaving the Senate", she said.

"I have achieved what I set out to achieve when I took over the leadership. The Greens have gone from strength to strength with solid election results and a growing, engaged party membership.

"I promised a more cabinet-style, collaborative approach to leadership. I am so proud of the way my colleagues have responded. We are a strong, capable, visionary Greens team.

"We have stood strongly for a safe climate and an end to wealth inequality. We have stood with the community against the cruelty of the Abbott government, with their first budget resoundingly rejected by the people, and the Senate."

She said she was looking forward to spending more time in Tasmania with her family, particularly because she was soon to become a grandmother.

"Life after Parliament is not however, life after politics. The fight for action on global warming will continue and I will take my passion, and all that I've learnt, to that fight standing shoulder to shoulder with the community here, and all over the world, for climate justice," Senator Milne said.

"I would like to say thank you. To my family, staff, colleagues, friends, party members, supporters and voters – thank you. I have cherished your input, your passion, and your support."

Prime Minister Tony Abbott paid tribute to Senator Milne.

"It takes a great deal of commitment and patriotism to offer to serve in the Parliament, to serve in senior
levels as Christine Milne has done," he said.

"It is a testament to her commitment to Australia. Even though we have far more often than not been on opposite sides of most political arguments, I respect her commitment.

"We've always had good and cordial relations. I understand that she's soon to be a grandmother and that's a marvellous time for any individual and, look, I wish her well in her future."

Opposition Leader Bill Shorten congratulated Senator Milne for a distinguished career in Australian politics.

"I haven't always agreed with every policy she's had ... but I can certainly respect the conviction and the passion with which she has advanced her issues," Mr Shorten said.

Former Greens leader Bob Brown said Senator Milne's resignation had come as a surprise to him.

"Christine's been a great friend, certainly a fantastic member of parliament and representative in both State and Federal Parliaments for Tasmania," he told the ABC.

"She's a big loss for the Greens, there's no doubt about that.

"But, that said, the Greens has a talent pool now that we've built up over the last 10 or 15 years and a very good leader will emerge out of that pack."

Mr Brown said he could not predict who the new leader would be.

"There's quite a number of them [who] would be extremely good and popular leaders and, you know, the Greens are big enough now to be able to renew on a regular basis," he said.

Employment Minister and fellow Tasmanian Senator Eric Abetz said that although he and Senator Milne had seldom agreed on policy, she had been "a steadying influence on radical elements in the Greens, especially social issues".

Treasurer Joe Hockey told a media conference he hoped change at the top meant the Greens would "see commonsense" when negotiating with the government.

Mr Hockey said one of the moves that had surprised him after the last budget was the party's decision to oppose a plan to increase fuel excise.

At the time, Senator Milne said the Greens would not support it because the money raised from the increase was to be invested in roads and not public transport.

"It was completely at odds with their base, completely at odds with their philosophy, completely at odds with their policy. So, frankly, I never understood why the Greens were opposing that policy which we had in last year's budget and which we will have in this year's budget," Mr Hockey said.

He said he hoped Senator Milne's stepping down was "a retirement for all the right reasons rather than anything else".

"I just say to whoever is the new leader of the Greens, please, please offer us some bipartisan support in the Senate that helps us to strengthen the Australian economy," Mr Hockey said.

"Ultimately if the Australian economy is strong we can spend the money on the environment but when the Australian economy is weak or when it's facing long-term spending challenges, ultimately the environment suffers along the way." 

Does it really matter who will lead the freedom hating @Greens? Their anti-family & community destroying policies remain #greenspill #auspol— Kevin Andrews (@kevinandrewsmp) May 6, 2015

Christine has been a fearless and courageous activist, politician and leader. Party has grown under her leadership. Sad to see her leave.— Senator Lee Rhiannon (@leerhiannon) May 6, 2015

Christine Milne has left an enormous legacy after 25 years in political life. It’s been an honour to work with you Christine.— Richard Di Natale (@RichardDiNatale) May 6, 2015

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