Same-sex marriage trigger undecided: Turnbull

Supporters of same-sex marriage. (AAP) Source: AAP

Malcolm Turnbull says he'll carefully consider the legal mechanism for allowing a same-sex marriage bill.

The prime minister has also moved to head off concern that the "yes" and "no" cases in a plebiscite - which he has promised would be held in the next term of a coalition government - could lead to a divisive and hurtful national debate.

The issue arose on Thursday when Liberal MP Warren Entsch called on Mr Turnbull to allow the parliament to amend the Marriage Act this term, but keep the change on hold until triggered by a majority "yes" vote in a plebiscite.

"Generally, a plebiscite was not generally binding," Mr Entsch said. "In this case, it would be binding and that would become law."

Mr Turnbull told the parliament the mechanism had yet to be decided by the cabinet.

"The mechanics for that plebiscite will be the subject of careful consideration by cabinet and then consideration by the party room," Mr Turnbull said.

But there should be no doubt that if the $150 million plebiscite passes, same-sex marriage will become legal.

"It is clear - every Australian will get a vote and that vote will be respected, and if the vote is carried, it will become law," Mr Turnbull said.

Former Abbott government minister Eric Abetz, who stands by the coalition's 2013 election promise not to legislate gay marriage, said Mr Entsch's suggestion is unhelpful.

"It seems a bit of a thought bubble and an ambush to boot," Senator Abetz said.

"As far as I'm concerned, it would be seen as a breach of our election policy."

Nationals MP Andrew Broad said any attempt by the parliament to determine a position before asking the people would be "frankly strange".

Australian Marriage Equality's Rodney Croome said there could be legal problems with Mr Entsch's idea. "I have certainly spoken to legal and constitutional experts who have expressed some concerns about whether it's even possible for the current parliament to effectively bind the next parliament by handing over legislative power (to the people)," he said.

Opposition Leader Bill Shorten, who has promised to legislate same-sex marriage within 100 days of a Labor government taking office, fears a plebiscite could be socially divisive.

"I don't think that is a genie we should let out of the bottle, where a whole lot of people are able to stigmatise each other," he said.

Mr Turnbull told the parliament he was disappointed in Mr Shorten's "lack of faith in the Australian people".

"If there are unruly voices heard, they will be drowned out by the common sense and the respect and the general humanity of our people," the prime minister said. 

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