George Brandis' legal foe has attempted to tear holes in the Attorney-General's argument that Barnaby Joyce should keep his spot in Federal Parliament, telling the High Court the logic is flawed.

Key points: Tony Windsor challenging parliamentary tenure of political nemesis Barnaby Joyce

Tony Windsor challenging parliamentary tenure of political nemesis Barnaby Joyce Mr Windsor's lawyer Justin Gleeson has been in a feud with George Brandis

Mr Windsor's lawyer Justin Gleeson has been in a feud with George Brandis Lawyers for Nick Xenophon argue he has right to sit in Senate

The court is sitting as the Court of Disputed Returns in Canberra, hearing arguments into whether members of the Citizenship Seven have breached the constitution because of their dual citizenship.

The politicians have been caught by the ban on foreign citizens sitting in parliament outlined in Section 44 of the constitution.

The Deputy Prime Minister is the only Lower House politician in the group, and his tenure in Parliament is being challenged by political nemesis Tony Windsor.

Mr Windsor has enlisted the legal expertise of Justin Gleeson SC, the former solicitor-general who spectacularly resigned last year after a bitter public feud with Mr Brandis.

Mr Gleeson's successor, Solicitor-General Stephen Donoghue QC, and Mr Joyce's lawyer Bret Walker SC, have both argued the Nationals leader had no way of knowing he was a citizen of both Australia and New Zealand, despite his father being born across the Tasman.

That argument is being challenged by Mr Gleeson, who told the court it did not matter if Mr Joyce never felt he was a New Zealander.

Rather, it was a simple fact he was and did not bother to check.

Meanwhile, lawyers for senator Nick Xenophon have urged the High Court to adopt the approach of UK authorities in not viewing him as a fully fledged British citizen.

The influential South Australian crossbencher was born in Adelaide, but his parents were born in Greece and Cyprus respectively.

Despite checking he was not citizen by descent of either of those nations, he later discovered his father's Cypriot heritage awarded him a sub-class of British citizenship — a vestige from a time when the Mediterranean island nation was under UK colonial rule.

Senator Xenophon was not in court, but his lawyer Andrew Tokley SC told the seven justices that his client's citizenship status did not even allow him right of entry or the ability to live in the UK.

Sorry, this video has expired The Citizenship Seven: How did we get here? ( Matthew Doran )

Mr Tokley argued the UK Government and even the European Union drew the distinction between British citizenship and the sub-class of citizenship Senator Xenophon held.

"[It] illustrates you have a body politic seeing a UK overseas citizen as not a British citizen," he told the court.

'Genealogical witch hunts'

Earlier, former Nationals minister Matt Canavan's lawyer argued Italian citizenship law was so murky, it was unclear whether his client was even an Italian subject.

David Bennett QC said various changes to Italian law had led to the situation currently before the court, and his client should not be punished by being kicked out of parliament.

Mr Bennett said some Italian legal experts suggested the current rules could allow "indefinite citizenship by descent" — even if an individual never sought to become Italian.

"Members of Parliament always have people desiring to have them removed," Mr Bennett argued.

He claimed allowing foreign laws to grant citizenship on people many generations after their families had travelled to Australia would lead to "people engaging in genealogical witch hunts that will occupy this court every time there is an election".

"Such offensive inquiries are totally inappropriate in a nation of immigrants, which was a nation of immigrants as well in 1901," he said.

Greens arguing against themselves

Former Greens senators Scott Ludlam and Larissa Waters are not challenging that their former dual citizenship status breached the constitution — even though the Commonwealth has argued the court should save Ms Waters' political career.

But the Greens' lawyer Brian Walters QC is arguing all of the Citizenship Seven should be shown the door through what he is claiming would be the most simple interpretation of the constitution.

The seven justices peppered Mr Walters with questions, before Chief Justice Susan Kiefel queried Mr Walters' motives.

"Is it a proper use of the court's time to argue for a vindication of [Ludlam and Waters'] view?"

Both quit Parliament as soon as they discovered they were dual citizens, describing it as the honourable course of action.