One Nation senator Malcolm Roberts has argued
it would be un-Australian to kick him out of Federal Parliament because of his dual citizenship, and that the UK should not have been considered a foreign power.
His lawyers were the last to appear before the seven justices, but Senator Roberts was the first of the Citizenship Seven to personally appear in court.
The court reserved its decision on citizenship, but noted the need for an urgent ruling.
Earlier, Senator Roberts dodged questions from the media as he arrived at the High Court in Canberra, flanked by an adviser.
Senator Roberts' barrister, Robert Newlinds SC, did not take long to begin a strident defence of his client, claiming the fact he was born overseas was almost irrelevant.
The One Nation senator was born in India to a Welsh father and Australian mother, moved to Australia when he was a teenager, and became an Australian citizen a short time later.
Mr Newlinds criticised the Commonwealth for trying to distinguish between the politicians born in Australia, referred to as "natural-born Australians", and those born overseas.
He said it would have been no different if Senator Roberts was born in "downtown Parkes, Canberra" because his father was Welsh.
The Commonwealth has argued those born overseas should have known there was a risk they were dual citizens.
"He's a natural-born Indian, whatever that means," Mr Newlinds told the court.
The court has heard arguments on whether Senator Roberts and six other politicians, including the Deputy Prime Minister Barnaby Joyce, breached the constitution because of their dual citizenship.
Section 44 of the constitution bans foreign nationals sitting in federal parliament.
Senator Roberts' lawyers argue his case is the strongest of the seven before the court, and there were similarities between his case and those of Barnaby Joyce and Deputy Nationals leader Fiona Nash.
"I'm trying to get myself in the same boat as Mr Joyce and Senator Nash, and then I want to demonstrate that I'm actually in a better boat," Mr Newlinds told the court.
Despite his almost comical attempts to ask UK authorities about his possible status as a British citizen, Mr Newlinds argued he at least had "the wit" to enquire about his status.
The other six politicians, Senator Roberts' lawyers argue, did nothing.
Mr Newlinds said when Senator Roberts became an Australian citizen he did not have to pledge his allegiance to Australia because he was already British, which was not considered a foreign power until 1986.
That argument seemed to frustrate the bench, with Justice Virginia Bell quick to point out the current case dealt with Senator Roberts' citizenship at the time of the 2016 election.
The exasperation of the bench continued, with questions seeming to suggest Mr Newlinds was taking too long to argue his case and dealing with matters that were not relevant.
If Senator Roberts' election at last July's federal poll is ruled invalid, it is likely a special recount of the Queensland Senate vote would be ordered.
The likely One Nation candidate to replace him is Fraser Anning, who has already declared his intention to take the seat if Senator Roberts is disqualified.
In the event Mr Joyce is kicked out of Parliament, a by-election would be ordered for his electorate of New England.
His political nemesis, Tony Windsor, is the only other likely contender in that battle.
Mr Windsor has enlisted former solicitor-general Justin Gleeson to challenge Mr Joyce's election.
Mr Gleeson spectacularly quit as the nation's chief lawyer last year, after a bitter public feud with Attorney-General George Brandis.