Despite his bombshell announcement, Mr Joyce is refusing to step down from cabinet or abstain from votes in the lower house - where the Coalition has a one seat majority - claiming he is confident the High Court will clear him to stay on.

However the Nationals leader is also taking urgent steps to renounce his New Zealand citizenship, paving the way for him to run again in case the court rules him ineligible and orders a byelection in his NSW seat of New England.

Mr Turnbull is also confident the court will clear his deputy, declaring: "The Deputy Prime Minister is qualified to sit in this house and the High Court will so hold."

But constitutional experts do not share the Prime Minister's confidence and Labor is questioning the government's entire legitimacy. The spotlight has also once again turned back on other foreign-born MP - or MPs with parents born overseas - including the Liberal lower house MP Julia Banks, who has Greek heritage.

In shock developments on Monday morning, Mr Joyce confessed to the dual citizenship concerns and referred himself to the High Court precisely one week after Fairfax Media first raised questions with his office and New Zealand authorities.

Mr Joyce's office refused to provide evidence of sole Australian citizenship and repeatedly refused to answer questions over recent days, before seeking advice from the government's Solicitor-General. Fairfax Media sent a final request for comment an hour before Mr Joyce's lower house bombshell.

Questions over dual citizenship - which is prohibited for members of parliament under section 44 of the constitution - have already forced two Greens senators to quit, Nationals senator Matt Canavan to resign as resources minister, and landed One Nation's Malcolm Roberts in the High Court.

Despite being born in Australia, Mr Joyce inherited his citizenship from his father - James Joyce - who was born in Dunedin in 1924. Under NZ law, anyone born to a NZ father between 1949 and 1978 is considered a "citizen by descent".

Mr Joyce said he was "shocked" to learn he was a dual Australian-New Zealand citizen.

"I was born in Tamworth, just as my mother and my great-grandma was born there 100 years earlier," he said.

"Neither I, nor my parents have ever had any reason to believe I may be a citizen of another country. I was born in Australia in 1967 to an Australian mother and think I am fifth generation. My father was born in New Zealand and came to Australia in 1947 as a British subject. In fact we were all British subjects at that time."

The government's advice is that he is unlikely to be disqualified: "However, to provide clarification to this very important area of the law, for this and future parliaments, I have asked the government to refer the matter," he said.

Soon after New Zealand Prime Minister Bill English personally confirmed Mr Joyce's status.

"Unwittingly or not, he's a New Zealand citizen," he said. "These things are almost always accidental. No one sets out to confuse the public with their citizenship."

Mr Joyce, who never registered his citizenship with New Zealand authorities, is expected to argue he had no way of knowing he was a dual citizen, having previously inquired to see if he was registered with NZ authorities and being told he wasn't.

But University of Sydney constitutional expert Anne Twomey said Mr Turnbull's declaration that the court "will" clear Mr Joyce was "not terribly helpful".

"You won't find any constitutional lawyers expressing that kind of confidence," she said. Ms Twomey also discovered she had New Zealand citizenship while researching the laws on Fairfax's behalf.

Senator Canavan stepped down from the ministry and has abstained from votes since realising he was an Italian citizen last month. The government claims the Mr Joyce's circumstances are different.

The worst-case scenario for the government now would be a byelection, with its popularity at such a low ebb.

Former independent MP Tony Windsor, who held the seat for more than a decade and challenged Mr Joyce at last year's election, wouldn't rule out another attempt if the citizenship fiasco triggered a byelection.

"It's not front of mind but you never rule anything out in life," Mr Windsor, a long-time foe of the Nationals leader, told Fairfax Media. "If he's ruled out, it's a whole new ball game."

Labor openly questioned the government's legitimacy, with frontbencher Tony Burke saying the government was clinging to power.

"This is a government without legitimacy," he said. "We've never had a government before, ever since Federation, that has had to go to the High Court because they just weren't sure if they had a majority."

Mr Turnbull wrote to Opposition Leader Bill Shorten on Monday, offering him the "opportunity" to refer any Labor MPs who may potentially fall foul of the same constitutional section.

"It is manifestly in the national interest that the High Court have the opportunity to clarify the limits on the operation of section 44 of the constitution," Mr Turnbull wrote.

But Labor declined the offer, with the ALP's National Secretary Noah Carroll issuing a statement saying it has an "extensive and exhaustive process for ensuring that every candidate satisfies all constitutional requirements".

"One part of this process is that candidates are required to declare the citizenship status of their parents and grandparents. Where there are any issues with respect to dual citizenship - potential or otherwise - candidates are required to take all reasonable steps to renounce and satisfy the requirements of section 44.

"We are confident every member of the Labor caucus has been properly elected."

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