The Turnbull government will once again take the plebiscite option to the Senate, but is ready to pursue a postal vote as a backup plan.

The Liberals decided to stick with their policy of a plebiscite on same-sex marriage at a special meeting last night, out-voting a group of seven pro-reform Liberals that included Warren Entsch.

The government also formulated a backup plan for the public vote to be held through the postal system if the Senate blocks the original bill again, as it did in November last year.

“I've got serious reservations about a postal plebiscite,” Mr Entsch said.

“If I’m not happy with the integrity of the process... I also reserve my right to be able to introduce a bill.”

Chief among those reservations is a threatened legal challenge. Same-sex marriage advocates have received legal advice that the postal vote may be unconstitutional, because the parliament has not voted to set aside the money to pay for it.

Mr Entsch said he was worried about voter turnout and representation, particularly for those in remote or rural communities, and young people who may be unfamiliar with the postal system.

Finance minister Mathias Cormann reaffirmed the postal plebiscite was the government’s backup plan.

“We're very confident that we have a legal and constitutional way forward to give the Australian people a say through a non-legislated postal plebiscite, yes,” Mr Cormann told reporters on Tuesday.

“If the answer is yes, the government will be facilitating consideration by the parliament of a private members bill to change the law to allow same-sex couples to marry.”

Mr Entsch said he was happy for the government to go through the process. “Good luck to them,” he said.

“I would like to have as many of my colleagues standing beside me when this vote is made.”

Labor has slammed the postal idea, with opposition leader Bill Shorten calling it a "colossal waste of time and money."

"There's a constitutional question over whether or not this postal opinion poll is even legitimate," Mr Shorten said.

"Why is it that the Liberal party spend all their agility and innovation on working out ways to delay marriage equality? I wish they'd put the same effort into electricity prices, I wish they'd put the same effort into sorting the banks out."

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Original plebiscite unlikely to pass as Xenophon, Lambie maintain opposition

The Coalition plans to get the original plebiscite before the Senate again before the week is out.

It is hoping some members of the crossbench, who were crucial in blocking the bill last year, will change their minds.

“We want to demonstrate to the Australian people that we honour our election commitment,” Dan Tehan, a Turnbull Government frontbencher, told ABC Radio on Tuesday morning.

But it is unlikely the bill will pass the Senate, as none of those who voted against it – including Labor, the Greens, and the Nick Xenophon bloc of three – have publically changed their minds.

Related Marriage equality backers vow to block postal plebiscite ahead of Liberal meeting

“Australians get two votes when they go to the polls. They get a vote for the lower house and the upper house. There are two mandates, and our mandate is to have this determined by the Parliament,” Nick Xenophon said.

Mr Entsch said he expected the plebiscite to be defeated again.

“I’ve got no doubt at all that it’s highly unlikely that will get through,” Warren Entsch, who was among the rebel Liberal MPs urging a free vote in the parliament.

Tasmanian crossbench senator Jacqui Lambie, who voted in favour of the plebscite last November, said she would not support a postal vote.

"Turnbull told voters he believed it was critical for everyone to have their say on any proposed change to the Marriage Act - and I agree," Senator Lambie wrote in a statement.

"A postal vote doesn't guarantee that. It's a deeply compromised position from a deeply compromised prime minister."

Senator Lambie said she held Malcolm Turnbull to account for failing to pass the original plebiscite, saying it showed "how poor" the government's negotations with the crossbench had been.