Australian Deputy Prime Minister Barnaby Joyce and four other Australian MPs have been ruled ineligible for election to parliament due to their dual nationality.
The High Court decision, delivered on Friday afternoon, means Australia's coalition government no longer has a majority in the House of Representatives.
Mr Joyce, who was born in Australia, but also inherited NZ citizenship through his Kiwi father, will now have to fight a by-election to regain his seat.
Australian parliamentary law prohibits MPs from holding dual citizenship and Mr Joyce referred himself to the High Court when he discovered the predicament. In August, he also announced he had renounced his NZ citizenship.
"We've received verbal communication from New Zealand... that that has now been accepted," he told Parliament.
"We're looking forward to the written advice turning up pronto."
Senators Malcolm Roberts, Larissa Waters, Fiona Nash and Scott Ludlam were also disqualified, with two others - Matt Canavan and Nick Xenophon - cleared of any impropriety.
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As soon as the High Court announced its ruling, Mr Joyce held a media conference in Tamworth, apologising to his constituents and promising a by-election as soon as possible - expected to be December 2.
"I was always apprehensive," he said. "I was always prepared for this outcome.
"In my gut, I thought this is the way it was going to go."
Because he has rennounced his NZ ties, Mr Joyce is now eligible to contest the by-election.
"Love me or hate me, I think I work pretty hard."
Opposition leader Bill Shorten tweeted soon afterwards: "Joyce broke the law and as a result, we now have a minority government.
"Turnbull should've stood him aside, terrible judgement again."
Mr Shorten's deputy, Tanya Plibersek, echoed her boss.
"We have a Prime Minister who has made these bad judgment calls again and again and again," she said.
Mr Turnbull says he is disappointed, and the High Court's decision will be referred to the Joint Standing Committee on Electoral Matters. There is the possibility of future changes to section 44 of the constitution.
The controversy caused a trans-Tasman spat when NZ Labour Party MP Chris Hipkins was accused of investigation the matter and began asking pointed questions in the NZ Parliament.
Mr Hipkins submitted a written question to the Minister of Internal Affairs: "Would a child born in Australia to a New Zealand father automatically have New Zealand citizenship?"
The enquiry played right into the hands of Australian media, who were already carrying out their own investigation.
At the time, Australian Foreign Minister Julie Bishop accused the Labour Party of conspiring to bring down her Government and warned that did not augur well if it was to win the then-upcoming election.
"New Zealand's facing an election," she said. "Should there be a change of government, I'd find it very hard to build trust with those involved in allegations designed to undermine the government of Australia."
The exchange caused some awkwardness last week, when NZ First leader Winston Peters - now our Foreign Affairs Minister - chose Labour as its coalition partner, installing Jacinda Ardern as NZ Prime Minister.