Same-sex marriage could be legal in Australia by next year under a new policy Liberal MPs have agreed on in an emergency party room meeting today.

Liberal backbenchers have overwhelmingly agreed to try for a second time to secure a plebiscite on marriage equality.

If that fails, the government will hold a postal plebiscite and then a free vote in parliament by the end of the year.

The government's first attempt at a plebiscite on marriage equality was blocked by Labor, Greens and crossbench senators in November.

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Prime Minister Turnbull called the emergency party meeting to secure a way forward, and bring Liberal infighting over the issue to an end, before Parliament returns tomorrow.

Marriage equality campaigners have already vowed to launch a legal challenge to a postal plebiscite if the government tries to hold one.

Sky News reports the government has received advice that it will be able to launch the postal vote without senate authorisation.

Earlier today, Nationals MP Andrew Broad threatened to quit the Coalition over same-sex marriage if the Liberal Party allowed MPs a parliamentary vote on the issue without a plebiscite.

Broad told his local newspaper, the Sunraysia Daily, if marriage law was changed without a plebiscite it would signal the end of the Coalition.

"It won't be me only, the whole show would blow up," he was quoted as saying. "So suddenly you'd lose 16 lower house members in one bloc. Turnbull's leadership would become untenable and he'd no longer be prime minister."

Labor leader Bill Shorten attacked Turnbull at a party meeting today, saying the marriage equality issue was dragging on because of his "complete failure of leadership and weakness".

"But there is only one reason - only one reason why this failure to deal with marriage equality has dragged on as long as it has. It is because of the complete failure of leadership and weakness of the Prime Minister of Australia to deal with this issue," he said.

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Labor agreed to back the bill if it was put before Parliament.

Liberal MPs Warren Entsch and Tim Wilson - who are among five MPs pushing for a parliamentary vote on a new marriage equality bill drafted by Liberal senator Dean Smith - said they would not rule out the plebiscite by mail option, which is set to be offered up as a compromise at today's meeting.

"I'm not going to rule anything out at this stage," Entsch told Sky News this morning.

"I'm going in there in good faith, let's have a respectful discussion on this, let's see what's on the table."

Entsch said a postal plebiscite was "fraught" but he wanted to see an outcome and a "clear end date" on the marriage equality debate.

Wilson told Sky News he would be "very open" to a postal plebiscite followed by a free vote as a "constructive" way forward.

Former Prime Minister Tony Abbott was among many who raised concerns about a postal vote.

"It's certainly better than ramming the thing through the parliament without any vote, but there could be questions about how authoritative it could be," Abbott told 2GB radio.

"There has to be a plebiscite. It would be very improper of us to abandon the clear commitment we took to the election for no change without a people's vote first."

Abbott issued a warning to colleagues pushing for a free vote on same-sex marriage this morning, insisting they were "honour-bound" to oppose it without a plebiscite.

"Keeping faith with the electorate should weigh just as heavily as deeply-held personal belief," he wrote in The Australian.

Labor MP Louise Pratt has also warned against a postal plebiscite, saying it could disenfranchise younger Australians who were more mobile than older Australians.

RELIGIOUS LEADERS BACK MARRIAGE EQUALITY

More than 500 Australian religious leaders have sent an open letter to Turnbull urging politicians to "show leadership" and legalise same-sex marriage.

Anglican, Catholic, Uniting Church, Hindu, Buddhist, Jewish and Muslim religious leaders have signed the letter.

"As people of faith, we understand that marriage is based on the values of love and commitment and we support civil marriage equality, not despite, but because of our faith and values," the letter reads.

It comes after Sydney Archbishop Anthony Fisher warned at the weekend that Senator Smith's bill would not protect Australians' religious freedoms.