The High Court could put Malcolm Turnbull's one-seat parliamentary majority in jeopardy if it disqualifies his deputy prime minister after a hearing on Barnaby Joyce's citizenship status starting on Tuesday.
Mr Joyce has New Zealand citizenship as a result of his father being born there, but did not realise until recently he could be a dual-citizen - which under section 44 of Australia's constitution renders a person ineligible to sit in parliament.
Mr Joyce has since renounced his NZ citizenship, but a court decision against him will force the government to fight a by-election in his NSW seat of New England.
However, the prime minister appears not to be contemplating a court loss.
"The government - based on the legal advice we have from the solicitor-general - is confident that the DPM and the other two senators, Nash and Canavan, will be found not to be disqualified from sitting in the parliament," Mr Turnbull told reporters in Sydney on Monday.
Constitutional law expert George Williams told AAP the key question for the court would be the meaning of the words "subject or a citizen ... of a foreign power", which in past cases has been interpreted by the court in a way which would not be favourable to the seven challenged parliamentarians.
In the 1992 Sykes v Cleary case, the court ruled two people could not stand for parliament despite them being ignorant of the fact that they were in breach of section 44 due to holding dual citizenship.
Two of Mr Joyce's Nationals colleagues, deputy leader Fiona Nash and former minister Matt Canavan, are also facing tests of their UK and Italian links respectively.
Experts believe the two most likely to be disqualified are One Nation's Malcolm Roberts and former Greens senator Scott Ludlam.
Senator Ludlam, who was born in New Zealand, resigned from parliament in July after finding he still held NZ citizenship.
India-born Senator Roberts was found by the High Court in September to have not properly renounced his UK citizenship by descent -through his Welsh father - at the time of his 2016 election nomination.
He is expected to be replaced by the next person on the party's election ticket, Fraser Anning, a businessman whose bankruptcy proceedings were dropped last week.
"It is very difficult to see that all seven can survive, given that the commonwealth itself is arguing that Senators Ludlum and Roberts should be disqualified," Prof Williams said.
Crossbench senator Nick Xenophon will be leaving parliament whatever happens, to contest the South Australian state election in March 2018.
If the court finds he's disqualified through his UK citizenship by descent, he will be replaced by the fourth candidate on the Nick Xenophon Team 2016 election ticket, Tim Storer.
If he is cleared, the NXT will pick the person who will fill his casual vacancy.
Former Greens senator Larissa Waters, who stood aside in July, will have her Canadian citizenship-by-birth tested in the court.
