Australians face a likely postal ballot on same-sex marriage after the Liberal Party quashed an internal push to drop its commitment to a plebiscite in favour of a conscience vote.
Liberal members attending a special meeting in Canberra on Monday stood by the policy taken to the 2016 election for a national vote on changing marriage laws.
The Turnbull government will resubmit its plebiscite plan to parliament this week.
If the bill fails a second time - which appears certain unless key crossbenchers flip their positions - a voluntary postal ballot would be conducted.
If the postal ballot comes back with a majority "yes", a private member's bill would go to parliament, with Liberal members exercising a free vote on it.
Cabinet minister Mathias Cormann said the government's preference was for a compulsory plebiscite, but if they cannot get it through the Senate, a voluntary postal vote would be held.
Crossbencher Derryn Hinch has already vowed to block the revived plebiscite push, saying the issue should be resolved through a conscience vote.
Senator Cormann said the government had advice there was a "legal and constitutional" way forward on the postal vote, but the specifics were a matter for the joint party room.
Queensland Liberal MP Warren Entsch, who was among Liberal MPs pushing for a free vote, says he reserves his right to take his own course of action on same-sex marriage but is happy to give a plebiscite another go.
"I'm not so much focused on the process. I want to get an outcome. And whatever it takes to get that outcome, I'm prepared to give it 100 per cent," he told the ABC's 7.30 program.
The Nationals have been staunch supporters of the plebiscite, with MP Andrew Broad warning the coalition could split if the policy was dumped.
They will join talks on the timing and mechanics of a potential postal vote during a coalition joint party room meeting in Canberra on Tuesday.
Advocacy group Australian Marriage Equality has legal advice it says confirms a postal vote would be unconstitutional, warning of a High Court challenge to prevent it going ahead.
Co-chair Alex Greenwich said the government's approach to marriage equality had gone "well beyond a joke".
"They had the opportunity to resolve this matter and they said no, they said we are going to drag this out," he said.
Opposition Leader Bill Shorten labelled the outcome ridiculous, saying he was disappointed for the hundreds of thousands of Australians the prime minister had let down again.