Malcolm Turnbull's government faces a month of uncertainty after the High Court disqualified two of his most senior ministers.
The court ruled that Deputy Prime Minister Barnaby Joyce and the Nationals deputy leader Fiona Nash were ineligible to stand for the 2016 election because they were dual citizens of New Zealand and Britain respectively.
Mr Joyce now faces a December 2 by-election in his NSW seat of New England, which he is widely expected to win with his arch-rival Tony Windsor pulling out.
But he won't be returning to parliament before February next year.
The court decision means the government will need the support of at least one crossbench MP on the floor of parliament's lower house for confidence, supply and any of its legislation.
Independent Cathy McGowan indicated on Friday she would support the government against any vote of no-confidence and for supply, but gave no guarantees on bills.
Labor says it's not planning any mischief in parliament, but cited weekend penalty rates and a royal commission into the banking sector - two issues it lost by one vote previously - as issues it would pursue when the House of Representatives returns on November 27.
The one bright note for the government was the return of Nationals senator Matt Canavan to cabinet as resources minister after the court ruled he was not disqualified for election.
"The decision of the court today is clearly not the outcome we were hoping for but the business of government goes on," the prime minister told reporters in Canberra on Friday.
Mr Turnbull has assumed the agriculture and water resources portfolio from Mr Joyce, with cabinet minister Nigel Scullion becoming interim parliamentary leader of the Nationals.
The prime minister is due to fly out of Australia on Saturday to attend the 100th anniversary of the Battle of Beersheba in Israel, and make a flying visit to Sri Lanka on the way back.
Foreign Minister Julie Bishop is expected to be acting prime minister if he goes.
The court also disqualified former Greens senators Scott Ludlam and Larissa Waters and One Nation senator Malcolm Roberts, but not independent Nick Xenophon.
Section 44 of Australia's constitution bans anyone holding dual citizenship from sitting in parliament, in a section aimed at ensuring MPs do not hold split allegiances.
Mr Joyce said the court's ruling did not surprise him.
"In my gut I thought this is the way it was going to go," he told reporters in Tamworth.
The prime minister had consistently expressed a view the court would support the government's argument his ministers were validly elected.
Labor deputy leader Tanya Plibersek seized on the decision, saying the prime minister had been "reckless" in allowing Mr Joyce and Senator Nash to retain their cabinet posts while the court decided their fate.
Labor has advice that decisions made by the two ministers and their colleague Senator Canavan since October 2016 could be challenged in court under section 64 of the constitution.
The section provides that "no minister of state shall hold office for a longer period than three months unless he is or becomes a senator or member of the House of Representatives".
Senator Roberts will run as a candidate for the Queensland seat of Ipswich at the state election.
Senator Xenophon plans to quit federal parliament in any case, to lead a team in the South Australian state election in March 2018
"The irony of the decision is not lost on me," he told reporters.
Independent MP Bob Katter described himself as "one of the six most powerful people in Australia at the moment".
A parliamentary committee will examine whether the constitution needs to be changed, or electoral processes tweaked, in light of the court decision.
