The High Court of Australia could deal a massive blow to Malcolm Turnbull's one-seat majority government, as it tests whether New Zealand "citizen by descent" Barnaby Joyce can continue to sit in parliament.
The deputy prime minister told parliament today he received advice from the New Zealand High Commission last Thursday he could be a citizen of New Zealand through his father who emigrated to Australia in 1947.
The Australian Constitution prohibits dual citizens from sitting in parliament.
New Zealand Prime Minister Bill English said his country's Internal Affairs Department confirmed Mr Joyce is a New Zealander.
"Unwittingly or not, he is a New Zealand citizen," Mr English told reporters.
"Then it's a matter for the Australian system to decide how Australian law applies in his case, and how they deal with the issue."
Mr Joyce was born in Tamworth in 1967 to an Australian mother and is a fifth generation Australian.
His father was born in New Zealand and went to Australia in 1947 as a British subject - a year before New Zealand citizenship was created as a legal concept.
"Neither my parents nor I have ever applied to register me as a New Zealand citizen. The New Zealand government has no register recognising me as a New Zealand citizen," Mr Joyce said.
The government has received legal advice from the solicitor-general which suggests Mr Joyce would not be found to be disqualified under section 44 of the constitution and would not have to resign as the Member for New England.
The government is expected to argue Mr Joyce had never been on a New Zealand citizen list, his parents had never applied for citizenship, he had no knowledge of the citizenship by descent and he had never sworn allegiance to New Zealand.
The High Court is now considering the futures of five MPs, with August 24 already set aside to hear cases involving Senator Canavan, former Greens Scott Ludlam and Larissa Waters and One Nation senator Malcolm Roberts.
Labor called on the prime minister not to accept the Nationals leader's vote and to stand him aside from cabinet until the court determined the matter.
But Mr Turnbull rejected the call and declined Labor's request to release legal advice.
"The leader of the Nationals party, the deputy prime minister, is qualified to sit in this house and the High Court will so hold," Mr Turnbull said.