If the change is supported by a majority of voters, the government will facilitate a vote on same-sex marriage before December 7, sparking the prospect of gay weddings before Christmas. If a majority of voters oppose change, there will be no parliamentary vote.

But so-called "rebel" Liberal MPs are already canvassing the possibility of ignoring a "No" vote if the turnout is low. It is understood they are eyeing a participation rate of 70 per cent for the poll to have legitimacy.

Liberal MP Warren Entsch said he would watch the process and the turnout, but reserved his right to put up a private member's bill even if the majority vote is "No".

Another Liberal MP, Trent Zimmerman, said he expected the turnout would be high, but a low turnout would "undermine the process", calling the result into question.

The government conceded the postal vote was the second-best option and will first ask the Senate to reconsider a compulsory attendance plebiscite, to be held on November 25.

If that is rejected, which is almost certain, it will bypass Parliament and enact the postal back-up, which it says does not need legislation. Ballots will open on September 12, with a decision finalised by November 15.

Prime Minister Malcolm Turnbull, who previously said a postal vote "flies in the face of Australian democracy", on Tuesday insisted the poll would be legitimate and was consistent with the Coalition's election commitment.

"Strong leaders carry out their promises. Weak leaders break them. I'm a strong leader," Mr Turnbull said. He reiterated his support for same-sex marriage but indicated he would not play a prominent role in the "yes" campaign.

"I have many other calls to my time," he said, naming national security, energy policy and the economy, and repeating his view that same-sex marriage is an obsession of the media.

The government says it has advice showing it can order the Australian Bureau of Statistics to survey the Australian people about same-sex marriage using a little-known provision of the budget. The ABS would be assisted by the Australian Electoral Commission.

Finance Minister Mathias Cormann, who is responsible for the plebiscite as acting Special Minister of State, said he would pay for the $122 million survey using the special Advance to the Finance Minister, a provision used just once in the past four years.

But the scheme will almost certainly face a High Court challenge from same-sex marriage advocates, who believe the government may lack the authority to conduct the ballot without legislation, and are seeking further legal advice.

Veteran gay rights campaigner Rodney Croome said he was "optimistic we can still make a case" to the High Court. But if the postal vote survives legal challenge, advocates will not boycott the campaign.

They retain concerns a voluntary postal vote is stacked against them, because younger voters would be less inclined to mail in their forms, and students who live away from their address on the electoral roll would be disadvantaged.

"At the moment it looks like a rigged game," said Equality Campaign executive director Tiernan Brady. "It is a dog's breakfast, but it has been designed to be a dog's breakfast."

Same-sex marriage opponents, including former prime minster Tony Abbott and Australian Christian Lobby director Lyle Shelton, were also concerned same-sex marriage supporters would not accept a "No" vote, particularly in a low turnout.

That view was lent legitimacy by some Liberal MPs who on Tuesday night indicated the poll's validity would hinge on the participation rate.

"I'll be looking at the process and the participation and a whole lot of other things," Mr Entsch told Fairfax Media. "If in my view, the outcome is flawed, I actually said today that I reserve my right to put up a [private member's] bill myself.

"I'm not going to make threats, I'm just saying I want to make sure of the integrity of the process."

Mr Zimmerman, another of the five "rebel" Liberals who had threatened to cross the floor, also indicated the turnout was crucial.

"Obviously if the turnout was very low, then that would undermine the process, but I don't expect that to be the case," he told Fairfax Media. "I'm expecting the postal vote will give us a clear outcome, and the Parliament will respect that outcome."

But the emphatic resolution of the Coalition party room to back the plebiscite and postal vote has killed off attempts by the "rebel" group to bring on an immediate conscience vote.

Liberal senator Dean Smith confirmed on Tuesday afternoon he will not seek to introduce his own private member's bill before a postal vote is held.

An opponent of a plebiscite in any form, Senator Smith said history would record the postal vote idea as "not the brightest moment in this government's history".