The Turnbull government is in crisis and its majority is on a knife-edge after the High Court disqualified deputy prime minister Barnaby Joyce from Parliament, sparking what is set to be a bruising byelection.
The court's verdict – which also knocked out deputy Nationals leader Fiona Nash – is a hammer blow for Prime Minister Malcolm Turnbull, who repeatedly expressed confidence his MPs would survive the challenge.
Labor is set to exploit the political uncertainty of the government's weakened hold on the floor of Parliament by attempting to reverse cuts to penalty rates and establish a royal commission into the banking sector, setting the stage for chaos when the lower house returns on November 27.
The byelection in the NSW seat New England will occur on December 2, with Mr Joyce tipped to prevail.
Late on Friday, Mr Turnbull's planned weekend trip to Israel was under review as he grappled with the fallout from the verdict. If he does persist with the trip, Foreign Minister Julie Bishop is likely to be acting prime minister.
The court ruled five federal politicians ineligible over their dual citizenship: Mr Joyce, Ms Nash, One Nation's Malcolm Roberts and former Greens senators Scott Ludlam and Larissa Waters.
It spared Nationals senator Matt Canavan – despite confusion about his Italian citizenship status – who was quickly sworn back into cabinet as Minister for Resources and Northern Australia. Mr Turnbull himself is taking over Mr Joyce's portfolio responsibilities of agriculture and water.
Mr Turnbull said he was disappointed with the result, but refused to answer questions about his earlier misplaced confidence.
"The decision of the court today is clearly not the outcome we were hoping for but the business of government goes on," he said. "We have a majority of members in the House of Representatives, even in the absence of Barnaby Joyce and, of course as you know, we have support from the crossbench."
With no one occupying Mr Joyce's seat, the government has lost its one-seat majority in the now-149 member lower house.
It has 74 seats on the floor of the house, while Labor and the crossbench will have 74 seats as well - meaning the government may have to rely on Speaker Tony Smith for his casting vote. It risks losing motions or being defeated on legislation if there is a single defector on an issue.
Crossbench MP Cathy McGowan has indicated she will continue to back the government on matters of confidence and supply.
After High Court-ordered upper house countbacks, Ms Nash is expected to be replaced by Liberal Hollie Hughes; Mr Roberts by Queensland publican Fraser Anning; Mr Ludlam by 22-year-old disability advocate Jordon Steele-John and Ms Waters by former Democrat Andrew Bartlett.
Mr Turnbull also announced a committee will review section 44 of the constitution - the clause that prohibits dual citizens serving in parliament.
Mr Joyce apologised to voters in his seat of New England for causing a byelection and indicated he had expected the court case could be lost - despite repeatedly assuring people he believed he was safe as a means to justify staying in cabinet while the court considered his case.
"I was always prepared for this outcome. I don't actually stand here totally surprised. I always expected that this was going to be a tough game," he said.
A Melbourne Cup-sized field of candidates – including from Labor, the Greens, One Nation and the Shooters and Fishers and Farmers – are tipped to run in the byelection. But Mr Joyce's bitter political rival, the former independent MP Tony Windsor who held the seat until 2013, has ruled himself out of the contest.
"I actually love elections, I've enjoyed the eight that I have contested but my wife doesn't and she had a pretty rough time last time with the tactics and strategies that were used not only against me," he said.
Mr Turnbull said Mr Joyce was "the best person" for New England.
"He has a passion for representation and while I know Barnaby will be disappointed with the outcome of the court case, it's as though he's been let out of the stalls and he's ready and raring to go," the Prime Minister said.
Labor is all but certain to use Mr Joyce's ousting to its political advantage when Parliament next sits for a week from November 27. The opposition has suggested it will not offer the government a "pair" during Mr Joyce's absence and deputy leader Tanya Plibersek suggested that every decision taken by Mr Joyce and Ms Nash as ministers was now under a legal cloud.
Ms Plibersek said it had been reckless of Mr Turnbull to leave Mr Joyce and Ms Nash in cabinet – rather than asking them to stand aside as Senator Canavan had done – but claimed Labor was not planning any "mischief". It would however examine closely decisions made by the Parliament, she said.
"I think one of the extraordinary revelations this afternoon is Barnaby Joyce thought himself ineligible, he said he was prepared for this outcome, and yet day after day in the Parliament he's been voting on legislation, some legislation that has passed only very narrowly with very serious consequences," she said.
"He's been making decisions as a minister; all of those decisions are now perhaps subject to legal challenge."
Senator Canavan released a statement saying he was "gutted for my colleagues Barnaby and Fiona" but he looked forward to "getting stuck back in to delivering long term results for all Queenslanders".
Communications Minister Mitch Fifield will take on Ms Nash's regional communications portfolio and Infrastructure Minister Darren Chester will add regional development and local government.
The court also spared crossbench senator Nick Xenophon but he has already announced he is leaving federal politics to pursue a state seat.
"This will be my last time seeing you as a senator. It's been an incredibly satisfying nine-and-a-half years," he said.
All the court's rulings were unanimous.
The court said it was adhering to the "ordinary textual meaning" of section 44 of the constitution in finding Mr Joyce and the four senators ineligible - effectively finding ignorance of one's status is no defence.
Adam Gartrell is the health and industrial relations correspondent for the Sydney Morning Herald and The Age, based in Parliament House