Greens senator Christine Milne has resigned as the leader of her party and will not recontest the next federal election.

Senator Milne made the shock announcement on social media, tweeting "feeling optimistic, proud & sad to announce I'm not contesting 2016 election, and so I resign as leader of Australian Greens".

In a statement, Senator Milne said she had made the decision with her family and informed her party-room colleagues of her decision this morning.

"After 25 years in politics, I am looking forward to spending more time in my beautiful home state of Tasmania, with friends and family, and especially as I am soon to be a grandmother," she said.

The party room is meeting to elect a new leader and Senator Richard di Natale has publicly declared he would throw his hat in the ring.

"I will be putting myself forward as a candidate to lead the Australian Greens," Senator Di Natale tweeted, adding "Christine Milne has left an enormous legacy after 25 years in political life. It's been an honour to work with you Christine".

The ABC understands some Greens, including the party's deputy leader Adam Bandt and senator Janet Rice, were not informed about the decision and found out through the media.

Former Greens leader Bob Brown said Senator Milne was a "big loss" for the party and that her resignation had come as a "big surprise" to him.

"She's just a legend, she's a fantastic Tasmanian, fantastic Australian and an international player," he said.

"But that said, the Greens have a talent pool now that has been built up for a number of years and a very good leader will emerge out of that pack."

Abbott praises Milne's record

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Both federal political leaders have paid tribute to Senator Milne, with Prime Minister Tony Abbott acknowledging her distinguished record.

"We come from very different political traditions but I respect Christine Milne as I respect all senior members of the Parliament," he said.

"It takes a great deal of commitment and patriotism to offer to serve in the Parliament."

Federal Opposition Leader Bill Shorten said while he has not always agreed with Senator Milne, he respected "the conviction with which she has advanced her issues".

The Government's leader in the Senate, and fellow Tasmanian, Eric Abetz, has offered Senator Milne his best wishes, adding she had made a significant contribution to public life.

"She was the first woman to lead a political party in Tasmania, and remains the only woman to lead a party at both a state and national level," he said.

Sorry, this video has expired The political journey of Christine Milne

"In spite of seldom agreeing with Senator Milne on many policy matters, I acknowledge that she has been a steadying influence on radical elements in the Greens, especially on social issues."

However, he added that he hoped the next Greens leader would reassess the party's "continual opposition" in the Senate to the Government's legislation.

That sentiment was shared by Federal Treasurer Joe Hockey who said he hoped the Greens would "see common sense".

"One of the things that surprised me about the Greens was that they would oppose an increase in the excise on fuel.

"It was completely at odds with their base, completely at odds with their philosophy, completely at odds with their policy."

Senator Milne was elected to the Australian Parliament in 2004 and took over as the party's leader from Bob Brown in 2012.

"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," she said.

The Tasmanian Greens will shortly begin pre-selecting candidates to contest the 2016 election, prompting Senator Milne's decision.