Coalition sets marriage policy: Turnbull

Paul Osborne, Belinda Merhab and Roje Adaimy
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The PM may use a secret ballot to determine where coalition MPs stand on marriage reform.

Bill Shorten says Labor will back a move by any federal politician to delay business in parliament if it leads to a vote on gay marriage.

The opposition leader on Thursday committed his party to supporting a suspension of standing orders so a private bill could be brought on.

"If it comes to that, yes," he told ABC's 7.30 program.

"If we get the chance to vote up marriage equality and have a vote on it in the parliament, a fair dinkum vote, we are absolutely up for that."

West Australian Liberal senator Dean Smith is working on a marriage equality bill, for which lower house colleagues Warren Entsch, Trevor Evans and Tim Wilson have flagged support.

But Malcolm Turnbull has poured cold water on the idea of the Liberal Party going it alone in legislating for same-sex marriage.

Nonetheless, he has called a special meeting of Liberal MPs in Canberra on Monday to discuss the issue ahead of a joint party room meeting on Tuesday.

The Nationals, meanwhile, are gathering separately in Rockhampton on Friday.

Mr Entsch - one of the Liberal MPs pushing for a vote in parliament rather than a national plebiscite - says it's vital Liberals are able to make a decision even if it runs contrary to that of their coalition partners.

He argues Liberal members were "ambushed" when former prime minister Tony Abbott brought the Nationals to a meeting on the issue in 2015, after which the coalition parties got behind the plebiscite as an election promise.

Asked about the issue in Perth on Thursday, Mr Turnbull told reporters: "Any issue can be discussed in the Liberal party room and the joint partyroom ... but of course coalition policy is ultimately determined in the coalition partyroom."

Mr Entsch said the plebiscite was a "captain's pick" by Mr Abbott and has become impossible to sell because some MPs say they won't accept the result.

"Instead of being ambushed and deceived, I trust this prime minister to do the right thing and give us an opportunity to have a fair hearing on this," Mr Entsch told Sky News.

He said the Nationals would be free to disagree with the Liberal position and vote against it.

The formal notification for Monday's Liberal meeting did not declare the agenda to solely be about same-sex marriage but one MP told AAP that "should go without saying".

Mr Turnbull is reportedly planning to use a secret ballot to canvass the opinions of his partyroom on the issue.

With the bill to enable a plebiscite failing to pass parliament last year, some Liberals want it re-committed to parliament, while others say a voluntary postal ballot could achieve the same aim without legislation.

Mr Abbott has warned that Liberal MPs crossing the floor would be engaging in a "serious attack" on Mr Turnbull's authority.

Mr Shorten said while Labor members are bound on process matters, they will be allowed a conscience vote on marriage equality.

He expects a "very small number" would vote against a bill to change the Marriage Act.