The fate of seven federal politicians, including Deputy Prime Minister Barnaby Joyce, will be decided by the High Court on Friday afternoon in Canberra.

Key points: Barnaby Joyce, Fiona Nash, Matt Canavan, Nick Xenophon, Malcolm Roberts, Scott Ludlam and Larissa Waters all face the possibility of being ruled ineligible to sit in parliament

Barnaby Joyce, Fiona Nash, Matt Canavan, Nick Xenophon, Malcolm Roberts, Scott Ludlam and Larissa Waters all face the possibility of being ruled ineligible to sit in parliament Senators Ludlam and Waters have already quit, while Xenophon is heading back to state politics

Senators Ludlam and Waters have already quit, while Xenophon is heading back to state politics The decisions on all seven politicians will be handed down at 2:15pm on Friday

The so-called Citizenship Seven are facing the possibility of being kicked out of Parliament because they were dual citizens at the time of the last election.

Section 44(1) of the constitution bans foreign nationals from election.

The politicians involved are Mr Joyce, deputy Nationals leader Fiona Nash, former Nationals minister Matt Canavan, South Australian senator Nick Xenophon, One Nation senator Malcolm Roberts and former Greens senators Scott Ludlam and Larissa Waters.

The Commonwealth argued the Deputy Prime Minister would never have known he was a dual citizen of Australia and New Zealand, and should not be punished.

Solicitor-General Stephen Donaghue QC, representing Attorney-General George Brandis, also argued the same position for senators Nash, Canavan and Xenophon.

The former Greens senators did not contest that they had breached the rules, and claimed their fellow politicians should also have been shown the door.

If Mr Joyce is kicked out, he will face a by-election in his New England electorate.

Most suggest he will retain the seat, amid what is likely to be a dirty campaign against the Deputy Prime Minister.

The senators involved would be replaced by special count of last year's Senate ballots.

Lawyers for Malcolm Roberts claimed his case to remain in the Upper House was the strongest, despite his almost comically misguided attempts to check with UK authorities if he was a British citizen, after being born in India to a Welsh father.

For Ms Waters and Mr Ludlam, the ruling will determine how they are replaced in the Senate, given they resigned immediately after revealing their foreign status.

Senator Xenophon has already flagged his intention to quit federal politics to run in the South Australian state election next March.

The seven justices of the High Court have been deliberating since October 12.

Chief Justice Susan Kiefel told lawyers for the seven politicians the court was well aware of the need for an urgent decision, but stressed she would not be rushed.

Federal Parliament is not sitting on Friday.