Tony Windsor, a one-time Independent MP, and Barnaby Joyce, the deputy prime minister, have been bitter rivals for many years.
That rivalry intensified when the two squared off in the northern New South Wales seat of New England at the last election.
The Deputy Prime Minister prevailed, but, with his dual New Zealand citizenship now revealed, Mr Windsor is arguing his opponent was not eligible to stand against him.
Mr Windsor has been allowed to join the High Court case which will determine whether Mr Joyce and at least four other politicians with dual citizenship were valid candidates.
Prime Minister Malcolm Turnbull maintains his confidence in the Government's legal advice that Mr Joyce will be cleared to stay in parliament.
"I'm sure that the court will clarify how section 44 operates, but I have to say again that we are very, very confident that our Members who have been caught up in this will be held by the court to be eligible to sit in the parliament and, therefore, eligible to be ministers."
National Party senator Matt Canavan, One Nation senator Malcolm Roberts and former Greens senators Scott Ludlam and Larissa Waters are the others in the case.
Independent senator Nick Xenophon and National Party deputy leader Fiona Nash have also revealed they hold British citizenship.
They are waiting for formal referral to the court.
Attorney-General George Brandis has defended the Commonwealth's decision to pay some of the legal expenses of those involved.
"It is the customary and long-established practice in cases like this that the Commonwealth pays the costs of people who were referred. There's a public interest in doing so. Obviously, the proper meaning and interpretation of section 44 of the Constitution is something which it is very much in the public interest to get clarified."
Solicitor-General Stephen Donaghue, QC, has told a directions hearing in Brisbane there is a clear demarcation line between the politicians.
He says there are those who had not known they were citizens of foreign powers and those who had.
He will argue that makes Barnaby Joyce's case different from those of Senator Roberts and of Senator Ludlam, who quit parliament after discovering his New Zealand citizenship.
Senator Canavan's barrister says he will produce evidence showing, if his client's Italian citizenship by descent rules him ineligible, half of Australia could not run for parliament.
University of Sydney constitutional lawyer Anne Twomey says the process will be laborious.
"We know that each of the relevant people who've been referred to the High Court gets a say, but the big question is, 'Well, who's the contradictor?' Who gets to say on the other side, 'Well, you know, I think that you are disqualified.'"
The Commonwealth unsuccessfully tried to have the hearing fast-tracked for next month.
But the court ruled it would consider the matter for three days in Canberra from October 10.
That will be after parliament's next round of sittings, when five of the seven will take their places despite the uncertainty about their political futures.
