JULIA Gillard will defy a push by Left faction powerbrokers for a formal position on gay marriage by backing calls to offer MPs a conscience vote.

The Prime Minister is now the only Labor leader in Australia opposed to gay marriage, with Queensland's Anna Bligh, NSW's John Robertson, Tasmania's Lara Giddings, Victoria's Daniel Andrews and the ACT's Katy Gallagher supporting change.

PHOTO: Finance Minister Senator Penny Wong said she would be among those backing the party's stance for marriage equality.

Senior ministers are declaring their support for gay marriage as the ALP prepares to back a free vote on same-sex marriage laws.

Finance Minister Penny Wong, who is expecting her first child with partner Sophie Allouache, wrote on the Rainbow Labor website: "I will be supporting change in the party's stance for marriage equality at the ALP's National Conference."

Tanya Plibersek said she would also push for change. Other frontbenchers who have declared they are not opposed to gay marriage include the Left's Anthony Albanese and Right's Mark Arbib.

Labor sources suggest Jenny Macklin and Nicola Roxon may also support change.

A conscience vote on gay marriage is now the expected compromise at the ALP conference in December.

The Left will still push for a clear statement of support for gay marriage in the ALP platform, but with the Right faction split on the issue a conscience vote is most likely.

Defence Minister Stephen Smith has declared that the majority view in his WA electorate is that "a marriage between a same-sex couple was something which was essentially a matter for them and the community or the state shouldn't interfere with that".

But the Australian Christian Lobby said the Prime Minister would break an election promise if laws are brought before this parliament because, during a taped interview, she vowed not to change the position.

"Her commitment was that marriage was between a man and a woman. If there's a private members' bill they should vote with the Coalition to vote it down," ACL chief Jim Wallace said.

The Australian Rainbow Labor Network said a conscience vote was not the answer. "Matters of equality should not be the subject of a conscience vote," spokesman Andrew Barr said.