PRIME Minister Malcolm Turnbull may be putting himself on a path of destruction after conceding that Liberal backbenchers "had the right to cross the floor" on controversial issues.

The Prime Minster's comments are being taken as a signal to MPs that same-sex marriage could be passed in the Parliament.

"It's always been a fundamental principle in the Liberal Party, and indeed the National Party," Mr Turnbull said.

"So it's a very different political culture to the very authoritarian and centrally controlled culture of the Labor Party."

Mr Turnbull took power from Tony Abbott with the support of the National Party, on the promise that same-sex marriage would never happen without a plebiscite, or national vote.
 

A private member's bill by WA Liberal Senator Dean Smith is likely to have the necessary support from Labor and the Greens.

Five Liberal MPs including gay Brisbane politician Trevor Evans have said they would support the changes to the law.

Only four would be needed for the bill to pass.

Powerful conservative elements of the Liberal Party are already warning that if the Prime Minister cannot stop colleagues from crossing the floor to vote against party policy, it could be "terminal".

Arch conservative and Immigration Minister Peter Dutton is thought to be the top choice to replace the PM, if indeed he was rolled.

Most western nations including New Zealand, Germany, France, the United States, United Kingdom and Ireland have already made same-sex marriage legal.
 

Labor's policy is to legalise same-sex marriage if it wins government at the next election.

In late June, Defence Minister and Turnbull ally Christopher Pyne was condemned by parts of the Liberal Party after being caught on tape saying marriage equality could happen "sooner than everyone thinks".

"We're going to get it. I think it might even be sooner than everyone thinks. And your friends in Canberra are working on that outcome."