Same-sex marriage could be legislated in Australia by the end of the year, despite second plebiscite push.

Same-sex marriage could be legislated by the end of the year, despite the government resubmitting its plebiscite bill to a likely lost vote in the parliament.

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.

However, if the bill fails a second time – which appears likely unless the Nick Xenophon Team changes its position – a postal ballot would be conducted.

Details were sketchy on Monday afternoon as to the format and legal basis for the postal vote, but it is understood the government has advice a postal vote is legal.

Before the meeting, same-sex marriage advocates released their own legal advice showing the government could not conduct a postal vote without its own legislation and that any move down that path would be open to challenge in the High Court.

Senior government members have been talking down the prospects of a private member’s bill, arguing the coalition promised no change would be made without Australians having their say.

“If we want to repair this trust deficit that people talk about in Australian politics, one of the first steps must be to allow the government of the day to keep its promises,” cabinet minister Arthur Sinodinos said.

Listen: Mia Freedman talks to Jayson Brunsdon and Aaron Elias, who are allowed to become fathers, but not get married.

The Labor caucus was briefed on WA Liberal senator Dean Smith’s private bill on Monday, agreeing that it represented an “acceptable compromise” and was in line with a Senate inquiry’s findings.

Labor MPs would get a conscience vote on it if the bill ever came to parliament, which is still possible if the postal ballot goes ahead and achieves a majority “yes” vote.

Labor frontbencher Terri Butler said it was disappointing the Liberal Party continued to put up more obstacles to marriage equality.

“The Liberal Party is already aware the will of the parliament is not to have a plebiscite, because the plebiscite legislation has already been defeated,” she told AAP.

“The Liberal Party in keeping with the sentiment of the electorate and the desire to do the right thing should seek to remove this discrimination against same-sex couples or at least seek to have a free vote on the floor of parliament, not recycle old ideas.”

The decision will now go to the joint coalition party room on Tuesday.

The Nationals have been staunch supporters of the plebiscite, with MP Andrew Broad warning the coalition could split if the policy was dumped.

Speaking before the meeting, Senator Smith said a postal vote was useless.

"It's a D-grade response to what is a defining A-grade social issue," Senator Smith said.

Former prime minister Tony Abbott said the postal vote was "certainly better than ramming the thing through the parliament", but he questioned whether it would carry the same authority as a plebiscite.

Advocacy group Australian Marriage Equality has legal advice it says confirms a postal vote would be unconstitutional.

According to the advice, the government does not have the power to spend money on a postal plebiscite without first passing legislation authorising use of taxpayer funds.

Advocates say they would seek an injunction to prevent the postal plebiscite from going ahead until a High Court decision on its constitutionality.