One Nation senator Malcolm Roberts' High Court bid to save his job relies in part on the notion it would be "fundamentally un-Australian" to discriminate between natural-born and immigrant Australians.
The Turnbull government faces a nervous wait after the court reserved its decision on the fate of the "citizenship seven" - which includes three government MPs - after three days of hearings in Canberra. Chief Justice Susan Kiefel said the bench would endeavour to deliver its rulings "as soon as possible".
On the final day of hearings on Thursday, the justices became impatient with the arguments presented by Senator Roberts' SC Robert Newlinds, and repeatedly asked him to hurry along.
Mr Newlinds argued the court should not accept the federal government's claim that there should be a distinction drawn between those MPs born in Australia and those born overseas like his client.
The government contends Senator Roberts and former Green senator Scott Ludlam have the weakest cases because they were born in India and New Zealand respectively. That means they should have been "on notice" as to the very real possibility they had dual citizenships.
The three government MPs before the court - Barnaby Joyce, Fiona Nash and Matt Canavan - were all born in Australia but inherited citizenship by descent from their parents. The government argues they could not take reasonable steps to renounce their foreign ties because they were ignorant of their status.
But despite representing a senator from a notoriously anti-immigration party, Mr Newlinds said the government's argument was a "fundamentally un-Australian notion".
???"There should be no place in Australian law - let alone Australian constitutional law - in 2016 for some dichotomy to be drawn between so-called natural-born Australians and immigrant Australians," he told the full bench. "If such a concept ever had a place in Australian law it should be put to bed."
The court has already found Senator Roberts was a British citizen at the time of his nomination for the Senate last year.
Senator Roberts was before the court because his father was from the UK, not because he was born in India, Mr Newlinds said. At one point calling Senator Roberts a "natural-born Indian", Mr Newlinds said the distinction between Australian-born and overseas-born MPs was just a "dangerous distraction".
Mr Newlinds said his client - who he described as "sensible and honest" - was in the "same boat" as people like Mr Joyce and Ms Nash - or an even "better boat".
"Mr Roberts is in exactly the same boat as people like Mr Joyce or Senator Nash - he didn't know he was a British citizen," he said. "But he had the wit to appreciate that he might be."
Mr Newlinds says Senator Roberts took reasonable steps to renounce - even though he did not follow the proper procedures and sent emails inquiring about his status to wrong or defunct addresses.
The High Court justices appeared to have little patience for Mr Newlinds' arguments, urging him to hurry along and challenging many of his assertions. At one point, Justice Bell said: "It is unclear to me where this argument is taking you."
Senator Roberts said after the hearing he was "very happy" with his case.
Earlier, the court heard from amicus curiae - or "friend of the court" - Geoffrey Kennett, SC. The independent court-appointed lawyer strongly backed many of the arguments raised by the legal teams of the Greens and Tony Windsor, Mr Joyce's political nemesis.
They believe all seven of the MPs before the court should be disqualified from office.
Mr Kennett said section 44 of the constitution was not just aimed at preventing divided loyalties, but the "risk or appearance" of divided loyalty. If ignorance was a permissible defence it created a perverse incentive for people to remain ignorant of their citizenship status, he suggested.
In comments that could signal big trouble for the government's case, Mr Kennett said it was essentially asking the court to rewrite section 44.