DEPUTY Prime Minister Barnaby Joyce has been ruled a Kiwi and ineligible to sit in the Australian Parliament, in a nation-stopping ruling by the High Court of Australia.
The High Court has also ruled Nationals deputy leader Fiona Nash, Greens senators Scott Ludlam and Clarissa Waters, and One Nation senator Malcolm Roberts ineligible.
However, Nationals senator Matt Canavan and crossbencher Nick Xenophon are eligible to continue to stay in Parliament.
Chief Justice Susan Kiefel read out the judgement to a packed, silent court.
Both the courtroom and overflow rooms in Canberra were full, with a sharp gasp when Justice Kiefel said Mr Joyce would have to face a by-election.
In a 44-page judgment, the court rejected arguments made by Attorney-General George Brandis, Mr Joyce and Senator Nash.
"Proof of a candidate's knowledge of his or her foreign citizenship status (or of facts that might put a candidate on inquiry as to the possibility he or she is a foreign citizen) is not necessary to bring about the disqualifying operation of s 44(i)," the judgement read.
The court, in its judgment, also said while some may suggest it was "harsh" to disqualify dual citizens who were born in Australia and never had a reason to consider themselves citizens of other countries, nominating for a federal election should be enough of a reason for them to check.
"Nomination for election is manifestly an occasion for serious reflection on this question; the nomination form for candidates for both the Senate and the House of Representatives requires candidates to declare that they are not rendered ineligible by s 44," the judgement reads.
Mr Joyce, Senator Nash, Senator Xenophon and Senator Canavan were born in Australia.
In August, Mr Joyce discovered he held New Zealand citizenship and took urgent steps to renounce it.
Mr Joyce, wearing his trademark Akubra hat, said a by-election in his seat of New England would be held around December 2.
"We're off to a by-election. Love me or hate me, I think I work pretty hard," he said.
"I was always apprehensive. I'm naturally cautious and I was always prepared for this outcome so I don't actually stand here totally surprised. I always expected this was going to be tough going.
"I respect the verdict of the court. I'll be making sure now we offer the people of New England the best services."
Mr Joyce said his deputy, Senator Nash, had been "stoic" during this tough period.
"It's great to see Senator Matt Canavan reinstated. Nick, he got back, but he still wants to go and obviously Ludlam and waters were struck out, like me."
Greens leader Richard Di Natale said Senators Waters and Ludlam were right to resign when they discovered they were dual citizens.
He said the Nationals politicians had been acting in self-interest.
"The easy thing to do was to act in their own narrow self-interest, like the Liberal and national party have done, to dig in, to ignore their constitutional advice and act like they are a law unto themselves.
"It has been particularly galling to see Malcolm Roberts sit in the Parliament for the past three months knowing he was ineligible."
New Zealand High Commissioner requested a meeting with Mr Joyce to tell him he was eligible for a Kiwi passport and entitled to citizenship because his father was born there.
Prime Minister Malcolm Turnbull chose to keep Mr Joyce within Cabinet and as deputy leader, saying he was confident the High Court would clear him.
He said the matter went to the "heart of our democracy".
Nationals senator Matt Canavan stood aside from Cabinet in July after discovering his mother applied for him to become an Italian citizen in 2006 when he was 25.
Mr Canavan initially argued the citizenship was obtained without his consent, so he should be able to remain in Parliament.
Then it was discovered he had been entitled to Italian citizenship since he was two years old, when the Italian constitution changed to allow citizenship by descent.
Born on the Gold Coast, Senator Canavan grew up in the Brisbane suburb of Logan and has never visited Italy or been into an Italian consulate before his citizenship scandal erupted.
The citizenship scandal unfolded after a lawyer in Perth discovered Greens senator Scott Ludlam was a New Zealand citizen.
This was quickly followed by the revelation his colleague, Greens senator Larissa Waters, held Canadian citizenship because she was born there.
Both resigned from Parliament immediately.
Ousted One Nation senator Malcolm Roberts is set to stand for the party in the Queensland election.
One Nation leader Pauline Hanson has announced "great man" Mr Roberts will stand in the seat of Ipswich in the Queensland election, which could be called as soon as this Sunday.
Labor frontbencher Anthony Albanese said Prime Minister Malcolm Turnbull no longer had a majority on the floor of Parliament.
"What we've seen is the government has lost its capacity to govern over recent months," Mr Albanese told Sky News.
"Today they've also lost their majority on the floor of the House of Representatives."
Asked what Labor would do if the government sought to suspend the Parliament until after a by-election in Mr Joyce's seat of New England, Mr Albanese said it would be "red hot".
HOW THE COURT RULED
● Deputy Prime Minister Barnaby Joyce, deputy Nationals leader Senator Fiona Nash, former Greens senators Scott Ludlam and Larissa Waters, and One Nation senator Malcolm Roberts were "a subject or a citizen ... of a foreign power" at the time of their nomination for the 2016 federal election.
● Each was therefore incapable of being chosen or of sitting as a senator or a member of the House of Representatives by reason of section 44(i) of the constitution.
● Nationals senator Matthew Canavan, who stood down from Cabinet, and independent Nick Xenophon were not disqualified from nomination for the election.
● Confirmed the ordinary textual meaning of s44(i) that an Australian citizen is not be prevented from standing for Parliament where it can be demonstrated that he or she took all steps reasonably required by foreign law to renounce his or her citizenship of a foreign power.
● Rejected the Commonwealth argument that s44(i) was subject to an implied mental element in relation to the acquisition or retention of foreign citizenship.
● There should be a by-election for the NSW seat of New England.
● The vacant Senate seats will be filled by a special count of ballot papers from the 2016 election.
WHAT WILL HAPPEN TO THE CITIZENSHIP SEVEN
●BARNABY JOYCE: With his election now ruled ineligible, Joyce will be forced to a by-election in his NSW seat of New England. He is likely to retain the seat. But the government, which has a majority of just one seat, will have six weeks of anxiety and distraction. Independent Tony Windsor could put up a solid fight if he decides to run again. A by-election loss would deliver minority government. Labor has advice his post-election ministerial decisions are open to legal challenge.
●FIONA NASH: Nash's Senate seat would ordinarily go to the next person on the NSW coalition ticket, who is this case is the Liberal Hollie Hughes. But the Nationals may insist on one of their own taking it, which would likely require Hughes to take up the seat, resign and cause a casual vacancy. Hughes is disqualified from sitting at the moment because she holds a government job at the Administrative Appeals Tribunal.
●MATT CANAVAN: He will remain in parliament and is expected to get his cabinet post back. If he lost, the next candidate on the Queensland Liberal- National Party ticket was Liberal Joanna Lindgren.
●MALCOLM ROBERTS: His seat will go to the next candidate on the Queensland One Nation ticket, Fraser Anning. Anning was facing bankruptcy proceedings which would have ruled him out of the seat, but the case has been dropped.
●NICK XENOPHON: Xenophon was going to quit the parliament anyway, but as he won the case, there will be a casual vacancy to be filled by NXT. At least three other senior NXT members are interested.
●LARISSA WATERS: Waters has already resigned from the Senate. Because she was disqualified she will be replaced by the next person on the Queensland Greens ticket, former Democrats senator Andrew Bartlett.
●SCOTT LUDLAM: Ludlam has also resigned. The next candidate on the West Australian Greens ticket is 23-year-old student Jordon Steele-John