THE country's highest court will be given data which could show up to half of Australians would be disqualified from entering Federal politics due to foreign citizenship by descent.
The High Court will rule on the eligibility of Deputy Prime Minister Barnaby Joyce, Senator Matt Canavan, One Nation Senator Malcolm Roberts and former Greens senators Scott Ludlam and Larissa Waters after a hearing in Canberra in October.
At a directions hearing in Brisbane today, Senator Canavan's barrister David Bennett QC said citizenship by descent "should be ignored by Section 44" of the constitution, which has wreaked havoc on Federal politics in the past month.
"We believe that we will be able to demonstrate if one applies Section 44 to citizenship by descent a very high proportion of the Australian population - possibly in the order of 50 per cent - would be disqualified," he said.
Mr Bennett said data from the Australian Bureau of Statistics would be provided to show the situation was "so ridiculous" that the section "should exclude citizenship by descent".
The barrister also revealed that Senator Canavan could have been an Italian citizen since he was a toddler, despite previously saying his mother signed him up 11 years ago.
Mr Bennett told the court that prior to 1983 Italian citizenship was only granted to the descendants of men in the country, but it was changed that year and the legislation was retrospective.
It meant Senator Canavan's mother - whose parents were Italian - automatically became a citizen, and so did he.
But Mr Bennett said the Southport-born politician was Italian by "the slenderest of connections".
Mr Joyce's barrister, Gerald Ng, said his evidence would centre around his "state of mind" as to whether he knew he was a citizen of New Zealand.
Former independent MP Tony Windsor will challenge Mr Joyce, and the court was told his lawyers would ask "what steps he did or did not take to ascertain if he was a citizen of a foreign power".
The court heard Senator Roberts' case was different and the evidence would centre around the steps he took to renounce his citizenship.
Solicitor-General Stephen Donaghue QC said Mr Ludlam's case was similar to Senator Roberts' as he was naturalised at the age of 19, so knew at some stage he was a New Zealand citizen.
Mr Donaghue said Ms Waters was in a similar position to Mr Joyce and Senator Canavan in that she did not know she was a Canadian citizen.
Chief Justice Susan Kiefel said the case had a "high degree of importance" and set it down for hearing on October 10, 11 and 12 in Canberra.
The cases of Senator Fiona Nash and Senator Nick Xenophon are expected to be referred to the court, sitting as the Court of Disputed Returns, before that time.
Justice Kiefel has also indicated she would agree with the Solicitor-General's call for all of the cases to be heard together.
She pointed out the cases arose in "unusual circumstances".
Attorney-General George Brandis has welcomed the High Court's decision to hear the cases together over three days in October.
Senator Brandis defended the government using taxpayer funds to pay the legal costs of MPs who were referred in such matters.
It was a long-established custom for the Commonwealth to foot the bill, he said.
Senator Brandis also defended Mr Joyce and other MPs for not knowing their citizenship status.
"I don't think it's anyone's fault," he said.
"Mr Joyce had no idea that there was an issue about his citizenship, neither did Senator Nash, neither did Senator Canavan, neither did Senator Xenophon. I think it's quite the wrong approach to lay any blame at anyone's door."
TURNBULL APPEARS UNFAZED
Malcolm Turnbull appeared unconcerned by the prospect of temporarily losing his one-seat majority if Mr Joyce is ruled ineligible to sit in parliament and a by-election is held in New England.
Speaking in Ettamogah today, the Prime Minister reiterated his confidence that Mr Joyce would retain his seat.
Mr Turnbull also said he had a "good relationship" with independent MP Cathy McGowan, who could provide confidence and supply to the government if it lost the majority in the lower house.
He said they were in frequent contact, but disagreed on whether Mr Joyce should retain his position in cabinet until the result of the High Court case was clear.
"She has indicated publicly she continued to support the government on confidence and supply," Mr Turnbull said.
"She has said that she believes that Barnaby is entitled to sit in the house and vote in the house, pending the High Court decision.
"She has suggested her view is that he should step down as a minister.
"I, obviously, respect her opinion, but my judgement is that he should stay as a minister because the only - because if you're entitled to sit in the parliament and vote in the parliament, then you're entitled to be a minister."
BARTLETT'S PUSH FOR GREENS SEAT
Earlier today, Greens' Andrew Bartlett - who is the most likely candidate to replace former senator Larissa Waters - said he was "keeping options open" when asked if he planned to take Ms Waters' seat and then stand down at a later time for her to replace him.
"I've talked with Larissa Waters a number of times, she is a good friend of mine, we have been communicating regularly ...," he said.
"That's a conversation plenty of people need to be involved in.
"I've said a few times, including to the Greens members, that I'm keeping options open ... there is a real opportunity for transformational change at the moment."
Mr Bartlett, who would be elected on a countback, said he was observing proceedings as an interested member of the public.
He reiterated that he was eligible to be elected despite holding a contract with the Australian National University when he nominated.
A section of the constitution saying a parliamentarian must not have held an "office of profit under the crown".
"I am confident on that, I have had legal advice on that multiple times," Mr Bartlett said.
Under section 44 of the constitution a person is incapable of being elected to the parliament if they are a "citizen or entitled to the rights or privileges of a subject or a citizen of a foreign power".
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