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. Credit: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.