The decision means that the seat belonging to Deputy Prime Minister Barnaby Joyce of the National Party, a coalition partner, will be subject to an election, which will be Dec. 2.
The ruling came months after what initially appeared to be a minor scandal forced several politicians to resign, eventually growing to present a challenge for the government of Prime Minister Malcolm Turnbull. The issue has raised questions about the vetting of candidates and how Australia’s Constitution is interpreted.
Under Section 44 of the Constitution, citizens of other countries are not eligible to run for the federal Parliament. Constitutional experts say the wording originally referred to people outside the Commonwealth, which included not only Australia but also Canada, New Zealand and a host of other British dominions and territories.
In a news conference, Mr. Joyce said he had always been prepared for the court ruling against him.
“It is a tough game, politics. You dedicate so much of your time to it,” he said. “You take the hits and the sacrifices.”
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Mr. Turnbull, who canceled a Saturday trip to Israel after the ruling came down Friday, said he was confident that Mr. Joyce would win the support of his constituents in the coming by-election, keeping his government’s hold on the lower chamber. Mr. Joyce formally renounced his dual citizenship after the scandal began and is now eligible to run.
“The decision of the court today is clearly not the outcome we were hoping for,” Mr. Turnbull said, “but the business of government goes on.”
The issue erupted in July when Scott Ludlam, a longtime Greens party senator from Western Australia, abruptly resigned after discovering he had retained New Zealand citizenship after becoming a naturalized Australian. While his supporters lamented the loss of a leader who had made a name with his stirring speeches and stern antinuclear stance, opponents accused the party of lackadaisical vetting.
Members of Australia’s most prominent parties began contacting foreign offices and consulates. To a mix of both broad amusement and dismay, five other senators — Matt Canavan of the Liberal National Party, Malcolm Roberts of the One Nation party, Fiona Nash of the Nationals, Nick Xenophon of the Nick Xenophon Team and Larissa Waters of the Australian Greens party — were found to have citizenship of other countries. So was Mr. Joyce, the deputy prime minister and House member.
On Friday, the court ruled that only Mr. Canavan and Mr. Xenophon had been eligible to run.
As the cases wound through the courts, the local news media named the group “the Citizenship Seven.”
Their situations varied. Some were born overseas and incorrectly believed they had renounced their citizenship rights. One senator, Mr. Xenophon, said he discovered that he held a rare type of British citizenship through his father, who left Cyprus before it gained independence from Britain. Another senator, Ms. Nash, said she was unaware she had been granted British citizenship by descent through her estranged Scottish father.
Mr. Joyce said he had not known he could be a citizen of New Zealand through his father until the local news media made inquiries to its Department of Internal Affairs.
Mr. Joyce’s odds of retaining his seat were bolstered after the ruling when his most formidable rival, Tony Windsor, declined to enter the race. But Mr. Joyce is likely to face other challengers.
Tanya Plibersek, the deputy leader of the opposition Labor Party, asked why Mr. Turnbull had allowed Mr. Joyce to stay in Parliament and make decisions despite facing the prospect that he would be declared ineligible.
“It is an extraordinary lapse of judgment from the prime minister,” she said.
Clive Bean, a professor of political science at the Queensland University of Technology, said Mr. Joyce’s party would probably keep the seat.
Still, voters may behave unpredictably in by-elections, he said. “Sometimes even a safe government seat may even be at risk because voters think, ‘Well, we’re going to send a message to the government in what we’re doing.’”
While the government would lose its formal majority in the lower chamber if Mr. Joyce does not win the by-election, it still has a tiebreaking vote in the speaker of the House, Tony Smith, a Liberal ally of Mr. Turnbull.
The government’s grip can be further solidified with the backing of independent members of Parliament. One such member, Cathy McGowan, has already said she would continue to vote in support.
The by-election is not the only concern Mr. Joyce faces, said Anne Twomey, a professor of constitutional law at the University of Sydney.
“There may be legal challenges to decisions he’s made in the past, and that might be problematic,” she said. “This is all uncharted territory.”
In the cases before the High Court, the solicitor general representing the Commonwealth, Stephen Donoghue, had argued that five of the politicians, most of whom inherited citizenship, could not have known they held foreign citizenship without expert knowledge of foreign legal systems in relation to their family roots. Once they were aware, he argued, they had made reasonable efforts to renounce foreign allegiances and thus should not be disqualified from office.
To Mr. Donoghue’s surprise, two of the politicians in the cases — Mr. Ludlam and Ms. Waters — argued that all seven should be disqualified. Both resigned their positions after their dual citizenship came to light. Mr. Xenophon also announced his resignation but for an unrelated reason: to run for the lower house of the State Parliament in South Australia.
Mr. Xenophon said that although he had won his case, he felt sympathy for his colleagues who lost.
“Whatever political differences I may have with some of them, there is no question of their love for or loyalty to Australia,” he said.