Greens senator Sarah Hanson-Young is facing a challenge from former senator Robert Simms for the prized top spot on the party's South Australian Senate ticket ahead of the next federal election.

Mr Simms today wrote to Greens members in South Australia, urging them to vote for him in the party's preselection process.

"Refreshing our Senate ticket provides us with an opportunity to reconnect with those South Australians who may have switched off from us," Mr Simms said.

"With high unemployment and rising inequality, SA voters are desperate for change.

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

Mr Simms ran second on the ticket behind Senator Hanson-Young at the 2016 double dissolution federal election.

Robert Simms (left) said "SA voters are desperate for change". ( ABC News: Leah MacLennan )

But with the Greens securing 5.87 per cent of the vote in South Australia, only one Greens senator was elected — meaning Mr Simms lost his spot in the Federal Parliament.

The number one spot on the Senate ticket will be decided by a ballot of Greens membership. Nominations are open until next week.

The ABC understands that while Senator Hanson-Young won the last closely fought preselection battle, attitudes may be shifting within the party to back another candidate.

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

Last year the senator adopted new branding on her email communications without reference to the Greens. ( Supplied )

Preselection opened last Monday, and closes at the end of this week. A vote of all SA Greens members will follow over April and May.

In a statement, Senator Hanson-Young defended her legacy and said it was "terrific" that the Greens allow any member to "throw their hat in the ring for preselection".

"Our members are the heart and soul of our party … I will be seeking their support to recontest the Senate race and represent them at the next election," she said.

"Preselection contests are the way that members ensure their senators are not just working hard, but working hard on the issues that matter to them to ensure we remain the home for progressive voters in South Australia.

"Greens members know I stand up for our state, our environment and our community. I will always stand up for, and with them."

Earlier this year, the NSW Greens voted to dump senator Lee Rhiannon from top spot on the party's Senate ticket in favour of state Upper House member Mehreen Faruqi, after a furious dispute with her colleagues which led to her temporary exclusion from the Greens party room.

Preselection battles have also flared in Victoria, as party members face allegations of sabotaging Greens candidate Alex Bhathal's campaign in the Batman by-election.

Mr Simms, a former Adelaide councillor, was selected by the Greens to replace former senator Penny Wright in a casual vacancy in 2015.

He ran as the Greens' candidate in the inner-city seat of Adelaide at the state election earlier this month, securing 12.6 per cent of the vote, a marginally better result than his previous attempt at the 2014 state election.

Robert Simms (C) replaced Penny Wright (R) in 2015, but lost his Senate seat at the 2016 election. ( ABC News: Tom Fedorowytsch )

Senator Hanson-Young's position in the federal Greens' partyroom has been under scrutiny for some time.

In late 2016, she expressed disappointment at the decision of party leader Richard Di Natale to strip her of the high-profile immigration portfolio.