Malcolm Turnbull and Christopher Pyne have rallied behind backbencher Warren Entsch as he introduced to parliament a cross-party bill to allow same-sex marriage.
Mr Pyne and Mr Turnbull were the only cabinet ministers in the House of Representatives chamber for the bill's introduction on Monday morning.
"(The bill) has the rare distinction of enjoying very strong cross-party support," Mr Entsch said.
The Queensland Liberal National Party MP was joined in the chamber by most of the Labor caucus, including Opposition Leader Bill Shorten, as well as independents, Greens and a handful of coalition colleagues.
The coalition party room last week rejected giving MPs a free vote on the bill after a lengthy debate, meaning any ministers would have to resign their portfolio if they want to vote for it.
Mr Entsch said the issue was about forging an "inclusive Australia, not a divided one".
"A divided nation is what we will be if we continue to allow discrimination in relation to marriage on the basis of a person's sexuality."
The MP later told reporters the bill still had a chance of going to a vote.
"I wouldn't be introducing it if I didn't hope we could have a vote on it," Mr Entsch said.
The decision on when the bill comes back for debate will be made by the parliament's selection committee.
Federal cabinet on Monday is expected to discuss the format for a plebiscite or referendum on the issue.
Mr Turnbull says a plebiscite should be held before the election - a move Mr Entsch supports.
Some ministers say the vote should be a referendum to change the definition of marriage in the constitution - which would be more likely to fail than a plebiscite.
Prime Minister Tony Abbott said the coalition party room had made a strong decision to "keep faith with the electorate" through this term of parliament, but put it to a vote in the next term.
"It should be the people's decision and that's what will happen in the next term of parliament," he said.
Mr Abbott said he respected Mr Entsch as a "terrific member of parliament" and a close friend.
Labor MP Terri Butler, who is seconding the bill, says the prime minister is doing everything in his power to stop same-sex marriage.
She doubts the government could organise a fair plebiscite, saying it's like wishing for the "sun to be blue".
"The $120 million on a plebiscite is $120 million of taxpayers' money being used by Tony Abbott to try to help block marriage equality," she told reporters in Canberra.
Greens senator Janet Rice says the minor party will continue to push the same-sex marriage debate in the upper house if the cross-party bill doesn't go anywhere in the lower house.
A Labor bill is also still before parliament and will be debated later on Monday.
