Australian Greens members are set to elect a new leader this morning after the shock resignation of party leader, Senator Christine Milne.

Greens health spokesman Richard Di Natale has confirmed he will contest the leadership ballot, set to take place Wednesday morning.

He is the only candidate to declare his intentions so far.

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The move reignites a long period of leadership speculation since a reported split amongst the party’s top performers.

In September 2013 Senator Sarah Hanson Young reportedly told a group of journalists the party was “marching to a slow death”.

I will be putting myself forward as a candidate to lead the Australian Greens

— Richard Di Natale (@RichardDiNatale) May 6, 2015

At the time Ms Hanson-Young, WA Senator Scott Ludlam and Melbourne MP Adam Bandt were favourites as challengers.

Feeling optimistic, proud & sad to announce I’m not contesting 2016 election, and so I resign as Leader of Australian Greens

— Christine Milne (@senatormilne) May 6, 2015

The exact reasons for Ms Milne’s resignation have not been made clear.

Former leader Bob Brown told ABC radio the party had plenty of depth from which it could select a new leader.

“There’s a wealth of talent there … and the Greens will come up with a very good representative.”

Ms Milne said in a statement she had “achieved what I set out to achieve when I took over the leadership”.

The Tasmanian Senator replaced Bob Brown as leader in April 2012 after serving as the deputy party leader for nearly four years from November 2008.

At the 2013 election the Greens brought in new MPs and the new leader, while their primary vote slumped.

Victorian Greens leader Greg Barber would not be drawn on leadership speculation and gave his tribute to the party’s outgoing federal leader.

“Tough, visionary and compassionate. Mind like a steel trap.The farmer’s daughter from Tasmania who became a global environmental leader,” he said.

Mr Di Natale also praised the former leader. 

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

A leadership ballot is due to be held Wednesday at 11.30 and the new leader and deputy leader are expected to be announced at a press conference at 1.30pm

Treasurer Joe Hockey called on the new Greens leader to offer the government some assistance in the Senate.

“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 strengthen the Australian economy,” Mr Hockey said.

Prime Minister Tony Abbott foreshadowed that Ms Milne may be resigning ahead of becoming a grandmother.

“Even though we have far more often than not been on opposite sides of most parliament arguments I respect her commitment,” he said

“I understand she is soon to be a grandmother, and that is a marvellous time for any individual. I wish her well in the future.”