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.

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

If the bill fails a second time - which appears certain unless key crossbenchers flip their positions - a voluntary postal ballot will be conducted.

If the postal ballot comes back with a majority 'yes', a private member's bill will go to parliament, with Liberal members exercising a free vote on it.

Crossbencher Derryn Hinch has already vowed to block the revived plebiscite push, saying the issue should be resolved through a conscience vote.

Cabinet minister Mathias Cormann says the government has advice there is a 'legal and constitutional' way forward on the postal vote, but the specifics are a matter for the joint party room which will meet on Tuesday.

Queensland Liberal MP Warren Entsch, who is 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.

Alex Greenwich (left) says the government's approach to marriage equality has gone 'well beyond a joke'

If the bill fails a second time - which appears certain unless key crossbenchers flip their positions - a voluntary postal ballot will be conducted

Victorian MP Jason Wood also prefers a parliamentary vote, but is willing to back a postal plebiscite.

'In recognising the concerns that my colleagues have, I'm more than happy to go with the postal vote option,' he told ABC radio on Tuesday.

His colleague Andrew Laming said those pushing for a free vote didn't need to be told to desist their campaign 'because they won't'.

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 says the government's approach to marriage equality has gone 'well beyond a joke'.

'It is now time for the government to get out of our way and have a vote on marriage equality,' he told the Nine Network.

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.