Hundreds of Sydneysiders have attended a rally supporting same-sex marriage, on the eve of a Liberal party room meeting on the issue. Picture: AAP

Labor leader Bill Shorten has attacked Mr Turnbull at a party meeting today, saying the marriage equality issue is 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.

Australia could have a way forward on same sex marriage by tonight with rogue Liberal MPs who were pushing for a free vote now saying they’re open to a postal plebiscite.

Australian Prime Minister Malcolm Turnbull will want to make a deal at the meeting. Picture: AAPSource:AAP

Gay couples could legally marry as early as next year if a postal plebiscite, followed by a free vote in Parliament within 100 days, is accepted by Liberal MPs at an emergency party room this afternoon.

But marriage equality campaigners have already labelled the postal vote a “bloody stupid idea” and are threatening to launch legal action to block any attempted plebiscite by mail.

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 the meeting at 4pm today.

Hundreds of Sydneysiders have attended a rally supporting same-sex marriage, on the eve of a Liberal party room meeting on the issue. Picture: AAPSource:AAP

“I’m not going to rule anything out at this stage,” Mr 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.”

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

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

RELIGIOUS LEADERS BACK MARRIAGE EQUALITY

More than 500 Australian religious leaders have sent an open letter to Prime Minister Malcolm 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.

Liberal MP's Warren Entsch and Trent Zimmerman are pushing for a parliamentary vote on Liberal backbencher Dean Smith’s marriage equality bill. Picture Kym SmithSource:News Corp Australia

POSTAL VOTE

The government believes a postal vote conducted by the Australian Electoral Commission could go ahead without a Senate vote to authorise the expense.

But co-chair of the equality campaign and director of legal advocacy at the Human Rights Law Centre, Anna Brown, has received legal advice that the government does not have the power to spend the funds without authorisation.

The group plans to launch a High Court challenge if the government announces a postal plebiscite.

“If that is announced today, the challenge to it will begin tomorrow,” Marriage Equality co-chair Alex Greenwich told reporters at Parliament House.

“A postal vote plebiscite is a bloody stupid idea.

“That will weaken the parliament because it basically says people are not prepared to do their job.”

Alex Greenwich from Australian Marriage Equality has labelled the postal vote idea ‘stupid’: Picture: Brook Mitchell/Getty ImagesSource:Getty Images

Former Prime Minister Tony Abbott has also 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,” Mr 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.”

Mr 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.

Former Prime Minister Tony Abbott has also raised concerns about a postal vote. Picture: AAPSource:AAP

“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.

Prime Minister Malcolm Turnbull has called the Liberal Party meeting for this afternoon in the hopes of securing a way forward on marriage equality before Parliament returns tomorrow.

Senator Smith’s bill and the postal plebiscite are expected to be discussed.

His proposal allows for two people to marry regardless of their gender, while enshrining protections and exemptions for religious communities.

That includes religious ministers, civil celebrants, and businesses linked to a religious body who refuse to cater for same-sex couples.

ReachTEL polling for The Australia Institute and Australian Marriage Equality shows almost two-thirds of voters in Mallee, a coalition stronghold, support MPs voting according to their own conscience on the issue.

More than two-thirds of voters in Mr Turnbull’s electorate, as well as the electorates of Mr Zimmerman, Mr Wilson, Mr Entsch and Mr Evans, also supported a conscience vote.

Meanwhile, UNSW constitutional law expert George Williams has warned a postal plebiscite would be dangerous and lack legitimacy.