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.
Joyce later apologised to his electoral division for the inconvenience of a by-election that will be held at the earliest possible date, December 2.
"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.
"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.
The disqualified senators included government minister Fiona Nash, Joyce's deputy in the Nationals party, who inherited British citizenship from her Scottish father.
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. Hewas 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.
The seven lawmakers said they did not know they were dual nationals when they ran for election last year.
Richard Di Natale, leader of the minor Greens party, praised the honesty and integrity of his former deputies Scott Ludlam and Larissa Waters, who resigned in July after discovering they were New Zealand and Canadian citizens respectively. None of the other five resigned.