The decision to disqualify Deputy Prime Minister Barnaby Joyce over a 116-year-old constitutional ban on dual citizens sitting in Parliament means a by-election will be held for his rural electoral district in December.
Prime Minister Malcolm Turnbull's conservative coalition has a single-seat majority in the 150-seat House of Representatives where parties form governments.
Joyce will be able to stand for re-election, having renounced the New Zealand citizenship he unknowingly inherited from his father. With the government trailing the opposition Labor Party in opinion polls, voters could use the by-election to toss both Joyce and his government out of office.
"I was always apprehensive, some people say I'm a natural pessimist, I'm naturally cautious and I was always prepared for this outcome," Joyce told reporters.
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"I'm going to make sure that I don't cry in my beer, I'm going to get back to work and work hard for the people of my electorate," he added.
The court also disqualified four of the six senators whose qualifications to be elected were debated in a three-day hearing earlier this month.
Another government minister Matt Canavan, who the court heard might have inherited Italian citizenship from his Australian-born mother through his Italian grandparents, was allowed to stay in Parliament.
Nick Xenophon was also allowed to stay in Parliament. He was born to Cypriot- and Greek-born parents and checked with both embassies to ensure he wasn't a citizen of those countries. He later found he was British because his father left Cyprus while it was a British colony.
Disqualified senators are replaced by members of their own parties without an election so the balance of power is not altered.