Members of Australia’s most prominent parties began contacting foreign offices and consulates. To a mix of both broad amusement and dismay, six other politicians — 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, Larissa Waters of the Australian Greens party and Mr. Joyce, the deputy prime minister — were found to have citizenship of other countries.
Friday’s ruling found that only Mr. Canavan and Mr. Xenophon were eligible to have run. And while Ms. Nash is also a member of the governing coalition, her seat will be filled by a member of the same ticket, so only the potential loss of Mr. Joyce’s seat could imperil the government.
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 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.
Scrutiny now will be on Mr. Joyce, who formally renounced his dual citizenship after the scandal began and is free to campaign to retain his seat. He may face a challenge from Tony Windsor, an independent politician who questioned Mr. Joyce’s eligibility for Parliament in the High Court case.
Clive Bean, a professor of political science at 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.’”
If another party gained the seat, the Turnbull government would have to court independent members of Parliament to pass legislation.
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 that they had made reasonable efforts to renounce foreign allegiances and thus should not be disqualified from office.
If the court had supported that argument, it would have made the standard for running for Parliament “much more subjective and more uncertain,” Ms. Twomey said.
She added, “How do you judge when a person has sufficient knowledge that there is considerable, sizable prospect of them being a foreign citizen?”
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 in the South Australian state election.
Ms. Waters said in an interview before the court ruling that she had mixed emotions. “I miss my job,” she said, while asserting that the Constitution was quite clear and that ignorance was not an excuse.
“You get one chance to do the right thing in this job,” she said. “If you don’t act with integrity you shouldn’t be there. I’m disappointed that others have not shown the same.”