It’s D-Day for the 'Citizenship Seven' - the federal politicians embroiled in the dual citizenship saga - with the decision threatening the government's one-seat majority should Barnaby Joyce be ruled ineligible.
Seven federal politicians, including Deputy Prime Minister Barnaby Joyce, will learn on Friday afternoon if they are eligible to sit in parliament in what will be a landmark decision in the High Court.
The decision could threaten the government's one-seat majority should Mr Joyce be ruled ineligible.
Prime Minister Malcolm Turnbull will also learn whether he will have to move for a by-election in Mr Joyce's northern NSW seat of New England.
The judgements on Nationals Senator Matt Canavan, Deputy Nationals leader Fiona Nash, South Australian Senator Nick Xenophon, former Greens Senators Larissa Waters and Scott Ludlam, and One Nation Senator Malcolm Roberts will be handed down after 2:15pm AEDT.
Section 44 of the Constitution prevents dual citizens from being a member of parliament.
During the High Court hearings, government lawyers argued five of them should be ruled eligible, except for Mr Ludlam and Senator Roberts, because they did not know they were dual citizens.
Solicitor-General Stephen Donaghue argued it was reasonable that where a person had no knowledge they ever were a foreign citizen, they should not need to take any steps to renounce their citizenship.
The government told the court the phrase 'is a subject or citizen ... of a foreign power' should be seen to refer only to a person who has voluntarily obtained or retained that status.
Mr Turnbull is travelling to Israel for a five-day trip on Friday for the Battle of Beersheba commemorations, raising questions as to who will be acting prime minister should Mr Joyce and Senator Nash be ruled ineligible.
There have been reports Deputy Liberal leader and Foreign Minister Julie Bishop would assume that role should there be adverse findings against the cabinet ministers.
Meanwhile Labor has received legal advice any decisions made by Mr Joyce, and Senators Nash and Canavan are open to a court challenge.
Section 64 of the constitution says 'no minister of state shall hold office for a longer period than three months unless he or she becomes a senator or member of the House of Representatives'.
Labor's advice is this would apply from the swearing-in date of a minister.
'Any decision made by the person in his or her purported capacity as a minister after three months has lapsed after his or her appointment as a minister is open to challenge,' the advice from Matthew Collins and Matthew Albert says.
The government has dismissed any concerns.
Ms Waters and Mr Ludlam have already resigned from parliament over their dual citizenship by birth in Canada and New Zealand respectively.
India-born One Nation Senator Roberts was found by the High Court in September not to have properly renounced his UK citizenship by descent - through his Welsh father - at the time of his 2016 election nomination.
Mr Joyce and Senators Canavan, Nash and Xenophon were born in Australia but were made citizens by descent.
Senator Xenophon, who found himself to have British citizenship by descent, will leave parliament whatever happens to contest the South Australian state election in March next year.
- With AAP