Former Nationals minister Matt Canavan will argue the total lack of clarity in Italian citizenship law means it would be grossly unfair to kick him out of Federal Parliament.
The "Citizenship Seven" had until this afternoon to submit their arguments to the High Court as to why their elections last July should not be declared void.
Submissions from lawyers for Senator Canavan, Deputy Prime Minister Barnaby Joyce, deputy Nationals leader Fiona Nash, South Australian senator Nick Xenophon, and former Greens senators Scott Ludlam and Larissa Waters have so far been made public by the High Court. Malcolm Roberts' submission was expected to be made available this evening.
All but the former Greens senators agree with arguments put forward earlier in the week by the Commonwealth that they had no way of knowing they were dual nationals, and therefore were not in breach of the constitution.
Ms Waters and Mr Ludlam are going as far as to dispute the Commonwealth's interpretation of the constitution, conceding they breached the rules and arguing the others probably have as well.
"There is no reason, as a matter of principle, to distinguish between Australian citizens who derive their citizenship by birth from those who derive it from descent," their lawyers said, claiming the Commonwealth was being tricky and arguing for a complex reading of the nation's founding document.
Lawyers for Senator Canavan have argued, even though he will not contest he was a dual citizen at the time of the last election, advice from Italian citizenship experts cast doubt on whether the nation's law even applied to him.
The Queensland Nationals senator has come under a constitutional cloud because his grandparents were born in Italy.
His lawyers are relying on the fact it was a retrospective change in Italian law that may have conferred citizenship on him at the tender age of two years old.
"Senator Canavan could not have been expected to have understood that in spite of his grandparents' naturalisation, in spite of having been born only an Australian citizen … and in spite of the need under that foreign law for 'paperwork' to be completed … he was nevertheless classified as a citizen under Italian law," they said.
Solicitor-general Stephen Donaghue QC, representing Senator Brandis, had argued the only two who should not be saved by the High Court were One Nation senator Malcolm Roberts and former Greens senator Scott Ludlam.
Xenophon 'never a subject or citizen of foreign power'
Senator Xenophon has ramped up the argument his tenure in Parliament should not be at risk because he unknowingly held a sub-class of UK citizenship — being a British Overseas Citizen (BOC).
His lawyers argued it was such an obscure form of citizenship that he never gained any benefit from it, and could not even live or travel freely to the UK.
"Thus at no time did Senator Xenophon enjoy, even under UK law, rights reflecting the minimum content of what it was to be a 'subject or citizen of a foreign power'," his lawyers argued.
"He enjoyed no right of entry and residence, no duty of allegiance, and no protection due from the UK Government — at least without registration.
"In consequence, Senator Xenophon was not … a 'subject or citizen of a foreign power'."