Malcolm Turnbull's parliamentary majority is in danger after the High Court disqualified Barnaby Joyce.
The deputy prime minister and Nationals leader will face a by-election in his seat of New England, expected on December 2, after the court on Friday found his New Zealand citizenship at the time of nomination in 2016 disqualified him.
Section 44 of the constitution prevents a subject or citizen of a foreign power from being chosen or sitting as a federal MP.
Since the 2016 election, Mr Turnbull has held onto power with a one-seat majority.
However, it's expected Mr Joyce will hold onto his seat - which has an 17 per cent margin - despite a potential challenge from former New England MP Tony Windsor and a host of minor party candidates.
Mr Joyce said just after the decision was announced he apologised for the inconvenience of a by-election.
"I respect the verdict of the court," Mr Joyce told reporters in Tamworth.
He said he was always apprehensive about the possible result and was not "totally surprised".
"In my gut I thought this is the way it was going to go," Mr Joyce said.
"I am going to make sure that I don't cry in my beer - I will get back at work and work hard for the people in my electorate ... and do the best for the nation."
He said he had no reason to believe he was a citizen of any other country.
The court found that at the time of his nomination in 2016 Mr Joyce was a New Zealand citizen by descent through his father, James Joyce, who immigrated in 1947.
However, the deputy prime minister only became aware of the possibility he might have NZ citizenship after media inquiries to his office in July.
The NZ high commission informed him that he was a NZ citizen under the law of that country on August 10, and he has since formally renounced the citizenship.
The government argued the phrase "is a subject or a citizen ... of a foreign power" should be seen as only referring to a person who has voluntarily obtained or retained that status.
Mr Windsor, who was a party to the case, argued Mr Joyce knew at the time of nomination his father was born in a country other than Australia, and he ought to have made enquiries and renounced any foreign citizenship held.
Asked whether he was still a cabinet minister, Mr Joyce said: "Right now I'm not even a candidate - you have to be endorsed to be a candidate."
It would be up to the prime minister to discuss parliamentary numbers with the federal opposition, he said.
