Australian Deputy Prime Minister Barnaby Joyce is also a citizen of New Zealand, the NZ Government has confirmed, as Labor ramps up pressure for the Nationals leader to resign from cabinet until his case is settled.
The Turnbull Government has referred its own deputy leader to the High Court over his duel citizenship.
Under section 44 of Australia's Constitution, dual citizens are ineligible to serve in parliament.
If he were disqualified, Turnbull Government's one-seat majority in the lower house would be threatened.
Mr Joyce broke the news himself on Monday, revealing the New Zealand High Commission had contacted him last week to advise he may be a New Zealand citizen.
"Needless to say, I was shocked to receive this information," he said.
"I've always been an Australian citizen born in Tamworth. Neither my or my parents had any reason to believe that I may be a citizen of any other country."
"The New Zealand Government has no record of registering me as a New Zealand citizen."
Later on Monday, that suspicion was confirmed by New Zealand's Prime Minister Bill English.
"Unwittingly or not, he's a New Zealand citizen," Mr English said.
New Zealand's Internal Affairs Minister Peter Dunne confirmed Mr Joyce had inherited his citizenship automatically by descent, on account of his Kiwi father.
"It's automatically passed on, I don't know whether he [Mr Joyce] knew or not," Mr Dunne reportedly Radio NZ.
"He says he didn't know, he says he was under the belief his father had renounced the New Zealand citizenship.
"But the fact is it is all irrelevant - if he was eligible to receive the citizenship at the time, under our legislation he does, regardless of his subsequent circumstances," he said.
Turnbull confident Joyce will survive legal challenge
Prime Minister Malcolm Turnbull said the government was "very confident" about its legal advice that Mr Joyce would survive the High Court process.
The government has received advice from the Solicitor-General on the matter.
The High Court is already considering the cases of two resigned Greens senators - Scott Ludlam and Larissa Waters - as well as LNP Senator Matt Canavan and One Nation Senator Malcolm Roberts, who are both remaining in their jobs until the cases have been heard.
Senator Ludlam was revealed as a New Zealand citizen, while Senator Waters discovered she was also Canadian.
The court is considering whether Matt Canavan, who stepped down as from his frontbench post as resources minister but not from the Senate, should be disqualified because of his dual Italian citizenship.
Malcolm Roberts has been accused of once holding British citizenship, though he claims he renounced it before he was elected.
Labor demands Joyce stand aside
Labor is ramping up pressure on Barnaby Joyce to follow the lead of his Coalition colleague Matt Canavan and resign from his ministry until his eligibility case is settled.
The opposition has also seized on the prospect that Mr Joyce's duel citizenship may threaten the Turnbull Government's slim hold on the majority.
Labor's Tony Burke said, "We don't know whether this government has a deputy prime minister eligible under the Australian Constitution, and we don't even know whether we have a majority government in this country."
Mr Burke said the government should force Mr Joyce to immediately resign from cabinet.
Mr Joyce said he was so confident in his legal grounding that he would continue to serve as deputy leader until the court decides his political fate.
"Given the the strength of the legal advice the government has received, the prime minister has asked that I remain deputy prime minister and retain my ministerial duties," Mr Joyce said.
Labor pressed the government on why Senator Canavan was expected to stand aside from ministerial duties, but the same did not apply to Mr Joyce.
"It's up to Matt Canavan, and I guess up to Barnaby Joyce, to explain what the distinction is between his case and Senator Canavan's case," Labor MP Anthony Albanese said.
Mr Turnbull has written to Opposition Leader Bill Shorten, offering Labor the "opportunity" to refer any of its own MPs or senators.
But Labor has again reasserted its confidence that every Labor member was properly elected.
New Zealand lawyers say Joyce's consent not required
Several New Zealand lawyers specialising in citizenship law told SBS World News that Mr Joyce would not need to sign a form or register for New Zealand citizenship to receive his rights through descent.
"It's very possible that Barnaby became a citizen, even though he never knew anything about it and has never registered or had a passport or anything like that," according to Anna Hood, an international law expert at the University of Auckland.
Migration lawyer Jack Cheng at the Queen City Law firm, also in Auckland, agreed that Mr Joyce would not need to consciously agree to become a citizen.
