Greens senator Sarah Hanson-Young has stared down an internal party challenge to secure the top spot on the party's Senate ticket at the next federal election.

Key points: Sarah Hanson-Young wins top spot on the Greens' Senate ticket by a close two candidate vote of 53-47

Sarah Hanson-Young wins top spot on the Greens' Senate ticket by a close two candidate vote of 53-47 Her main competition was Robert Simms - a former Adelaide City councillor and Senator

Her main competition was Robert Simms - a former Adelaide City councillor and Senator Robert Simms is pushing for renewal of the Greens to bring people back to the party

Her win comes despite a strong push from Greens member Rob Simms, who wrote to members in South Australia in March urging them to vote for him in the party's pre-selection process.

The ABC understands the vote was close, with Senator Hanson-Young winning the preselection vote ahead of Mr Simms on a 53-47 two-candidate preferred basis.

Mr Simms, who was parachuted into the Senate in September 2015 following the resignation of senior Green Penny Wright, lost his position in the 2016 double dissolution election after running second on the ticket behind Senator Hanson-Young.

In a statement, he congratulated Senator Hanson-Young for her preselection.

"This preselection was a lively contest of ideas that engaged a record number of our members and I'm proud to have been able to play my part," Mr Simms said.

"Participatory democracy is one of the four pillars of the Greens and I believe our democratic culture has been strengthened as a result of this process.

"I want to thank the many Greens members who voted for me in this ballot and have supported me throughout my political journey."

Robert Simms lost the preselection battle to Greens senator Sarah Hanson-Young in a close race. ( ABC News: Tom Fedorowytsch )

Senator Hanson-Young also released a statement, saying she was "delighted" to be re-endorsed by the state's members.

Ms Hanson-Young's position within the federal Greens' partyroom has been under scrutiny in recent times, particularly after Greens leader Richard Di Natale stripped her of the high-profile immigration portfolio in late 2016.

Sources have alleged Senator Hanson-Young did not help the party during the state election campaign earlier this year, and was overseas at World Economic Forum events in Davos, Switzerland and Harvard University in the United States.

When questioned whether she ever feared losing the top spot on the party's ticket, she said "no".

"Together, the Greens team and the SA community will fight to protect our environment, to care for people and to support our local artists and creators," Ms Hanson-Young said.

"We will fight to save the Murray, to stop drilling in the Great Australian Bight, to take action on climate change and increase our investment in renewable energy."

In his letter to party members in March, Mr Simms appealed for a refresh of the Senate ticket, saying it was an opportunity to reconnect with South Australians who had switched off from the Greens.

"Our state is ripe for a progressive revolution and the Greens are the party to deliver it. This should be our time," he said.

The Greens have announced Ngarrindjeri elder Major 'Moogy' Sumner will be the party's candidate for Mayo, also being contested by former Centre Alliance MP Rebekha Sharkie and Liberal Georgina Downer.