Greens senator Sarah Hanson-Young says she is "horrified" at the prospect that her Senate spot could be taken by One Nation, as the Greens face losing two of their most high-profile parliamentarians at the upcoming election to Pauline Hanson's party.

South Australian senator Sarah Hanson-Young, pictured with Greens leader Richard Di Natale on Wednesday, is aiming to be elected for her fourth term. AAP

Along with Senator Hanson-Young, the Greens' co-deputy leader Larissa Waters has an uphill struggle to hold on to her Queensland seat due to challenges from One Nation, Clive Palmer and far-right senator Fraser Anning.

The Greens have six of their nine senators up for re-election on May 18. "Our focus is on retaining our team of MPs," Greens leader Richard Di Natale told The Sydney Morning Herald and The Age, saying South Australia, Queensland and New South Wales would "come down to the wire".

Senator Hanson-Young said she was fighting for one of the last two Senate spots in South Australia with the Centre Alliance's Skye Kakoschke-Moore (who was forced to resign over dual citizenship in 2018), One Nation candidate Jennifer Game and the Liberal Party's third candidate, Alex Antic.

Senator Hanson-Young, who was first elected to the Senate in 2007 and is now seeking a fourth term, said the contest was "on a knife edge".

"It horrifies me that if I lose my seat, it could be taken by a One Nation candidate. I'm going to fight tooth and nail to make sure they don't get one."

Ms Hanson campaigned in South Australia last week with Ms Game, who is a former tax officer and adviser to the One Nation leader. While the Greens are calling for a 100 per cent switch to renewable energy, One Nation wants to see a new low-emission coal-fired power plant in South Australia.

Senator Waters said she was vying for the last Senate spot in Queensland with One Nation's Malcolm Roberts, Mr Palmer, Senator Anning, prominent same-sex marriage opponent Lyle Shelton (running for Cory Bernardi's Australian Conservatives) and Katter's Australian Party.

"Long story short … it's me or those five blokes," she said.

Senator Waters, who was first elected in 2010, had an unplanned break from politics in 2017 when she discovered she was a Canadian citizen and resigned from Parliament. She returned to the Senate in September 2018 after her replacement, Andrew Bartlett, resigned to contest the lower house seat of Brisbane for the Greens.

Larissa Waters says she does not know if her time out from politics, due to the citizenship debacle, will hurt or help her campaign. Alex Ellinghausen

Senator Waters said she did not know whether the break had helped or hindered her chances.

"We'll all find out on polling day. People seem to know me a bit more than previous campaigns," she said, but noted this could be due to the citizenship debacle or the historic images of her breastfeeding her new baby in the Senate chamber in 2017.

Along with a strong emphasis on climate change action, senators Waters and Hanson-Young are both pitching their election contests as a fight for control of the Senate. It is possible Labor and the Greens will reach the 39 seats required to pass legislation, although other projections put One Nation or Centre Alliance senators in the driver's seat.

"The challenge for us is in this election is helping people focus on the importance of the Senate. It's crucial," Senator Hanson-Young said.

"Both here [in South Australia] and in Queensland, it's going to come down to us or them."

Election analyst Kevin Bonham described the South Australian contest as "possibly the diciest" for the party, given the Greens' vote in the state was not very high, but added that Senator Hanson-Young was not a write-off.

While unconvinced about the chances of Senator Anning and the Australian Conservatives in Queensland - based on previous election results - Mr Bonham said Senator Waters would also be competing against the third Liberal National Party candidate, Gerard Rennick, if there was not a big swing against the Coalition in the state.

"So long as there's a swing to Labor, I think the Greens should be OK in Queensland," Mr Bonham said.