South Australian Premier Steven Marshall "wouldn't be surprised" if Nick Xenophon nominated as a candidate for the Mayo by-election.
Centre Alliance MP for Mayo Rebekha Sharkie was one of four MPs who quit federal parliament on Wednesday after the High Court disqualified Labor senator Katy Gallagher - but all have vowed to recontest their seats.
"I suppose the big question for the people of South Australia is, is Nick Xenophon going to resurface at the by-election coming up?" Mr Marshall told reporters.
"Anything could happen with Nick Xenophon and his party."
But Mr Xenophon, who is taking a break from public life following his March state election defeat, took to Twitter on Wednesday afternoon to throw his support behind Ms Sharkie.
"Rebekha Sharkie is a fantastic advocate for the people of Mayo and thoroughly deserves to be re-elected!" he wrote.
"She has my 100 per cent support in the upcoming by-election."
Mr Marshall confirmed the Liberal party would stand a candidate at the by-election but would not say whether it would be Georgina Downer, the daughter of former member Alexander Downer.
"We're going to be working very hard to win back the seat of Mayo," he said.
"This has historically been in the Liberal fold."
Mr Xenophon has been contacted for comment.