Former prime minister Kevin Rudd says a "personal journey" has led him to change his mind and support same-sex marriage - a position that puts him at odds with his leader, Julia Gillard.

Ms Gillard does not support same-sex marriage, although she has allowed Labor MPs a conscience vote on the issue.

Mr Rudd says he has changed his mind after a "lot of reflection" and discussions with friends and family, as well as a former staff member who is gay.

"It was time to explain to the people of Brisbane who vote for me, and more broadly to people interested around the nation, the reasons for my change of position on this," Mr Rudd said.

"Surely Australia is grown up enough in the year 2013 for the secular state to have its definition of marriage, religious institutions and the church to have their definition of marriage, and for each of them to be able to conduct their own ceremonies.

"That's it in a nutshell."

Using the quote "to thine own self be true", Mr Rudd says he does not intend to become a "national campaigner" on the issue and will not try to sway the Prime Minister.

"Each person arrives at their position in this as a matter of conscience," he said.

"I'm sure in the case of the Prime Minister, she's arrived at her position of opposition to same-sex marriage as a product of her own conscience and her own deliberations. So that's a matter for her."

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Ms Gillard says Mr Rudd has simply used the opportunity provided by a conscience vote to come to his own conclusion.

"Obviously individual Labor members, including Kevin Rudd, will use that freedom to reflect deeply and to form their own views about same-sex marriage," she said.

"Kevin's done that; every Labor member is free to do that."

Both Ms Gillard and Mr Rudd repeated Labor's call for Opposition Leader Tony Abbott to allow a conscience vote for Coalition MPs.

However, Mr Abbott, whose sister is gay, says his position has not changed.

"I respect Kevin Rudd, I accept that he's entitled to change his mind. I certainly haven't changed my mind," he said.

"We took a particular policy into the last election. The policy that we took into the last election is that we support the existing Marriage Act.

"My party room was strongly of the view that we were not going to say one thing before an election and do the opposite after an election."

Brandis says decision politically motivated

Mr Abbott has rejected the notion that the former prime minister has made the announcement as part of another push to take back the Labor leadership, saying he does not want to be "cynical about something like this".

But Liberal senator George Brandis believes that is exactly Mr Rudd's motivation.

"The Rudd supporters in the federal caucus have not given up hope," he told Sky News.

"Every time they see a depressing opinion poll result they're at it again - the take out of this story, it's not about same-sex marriage.

"What it tells you is that Kevin Rudd has not given up; Kevin Rudd is at it again."

However, one of Mr Rudd's key supporters in caucus, Labor MP Joel Fitzgibbon, says people should not read too much into the announcement.

"He's obviously been struggling with the issue for a long time, as have all of us," he told Channel Seven.

"Indeed it's a tough topic in Canberra and I'm not all that surprised to know he's come to a conclusion."

Rudd said timing of announcement important

Mr Rudd says he deliberately timed his declaration of support for same-sex marriage because Greens legislation will soon be up for a vote.

"We have a vote in the House of Representatives on this in two to three weeks' time," he said.

"For me to have said anything prior to the budget would have been wrong, because it would have taken attention off the budget.

"And immediately after the budget when you are dealing with the sell of the budget ... would have been wrong as well.

"I've waited frankly for a bit of a lull, and also given that we've got a vote coming up and the debate's about to heat up soon, I have an obligation to explain to people what position I would take to that vote and why."

The Greens, who have welcomed Mr Rudd's change of heart, have a bill to support same-sex marriage before Parliament and are pushing for a vote this term.

Greens MP Adam Bandt introduced the bill in February 2012 and says he would be "very surprised" if there was not a vote this sitting period.

"The normal practice would be for a vote on June 6," he said. "This bill has probably been one of the most debated bills in this parliament.

"We'll be having more debate on my bill next week. I would be very surprised if Labor steps in and tries to block a vote."

However, Leader of the house Anthony Albanese, says it is unlikely there will be another vote on same sex marriage before the September 14 election.

Mr Albanese says any vote would fail unless Coalition MPs are granted a conscience vote.

He says that might see marriage equality set back.

"What I want to see is real change, not a defeated bill again - for a second time during this term," he told PM.

He says until recently the Greens did not want a vote on its bill for the same reason.

Labor backbencher Stephen Jones, whose private members bill to change the Marriage Act failed last year, and which Mr Rudd voted against, says he welcomes the former prime minister's support.

"I suspect there'll be a lot more people who make similar announcements over the not-too-distant future," Mr Jones told The World Today.

Christian Lobby says Rudd guided by 'shallow debate'

The Australian Christian Lobby (ACL) says Mr Rudd's new stance is "very disappointing" and accused him of being guided by the current "shallow debate".

"It seems that he hasn't fully considered the consequences of redefining marriage, and it seems like he's a victim of popular opinion rather than looking beyond the slogans by which this debate has been conducted over the last couple of years," ACL managing director Lyle Shelton said.

Sorry, this video has expired Australian Christian Lobby responds to Rudd's stance

"For a prime minister who rightly gave an apology to the Stolen Generations, he's sadly not thought through the fact that his new position on redefining marriage will create another stolen generation because marriage is a compound right to form a family.

"And this means that same-sex couples would need to be allowed to acquire children through surrogacy and other means of assisted reproductive technology - severing a child from its biological heritage."

Cabinet minister Penny Wong, who is gay and has a child, has blasted the ACL as "peddling prejudice".

"This sort of bigotry really has no place in modern Australia," she said.

"Same-sex families exist and we have children and our children are loved and our children are cherished."

Ms Gillard said in April that she opposed same-sex marriage because "we should develop and purpose-create something new that guides us for the future, so that there would be marriage in its very traditional form and then there would be other ways of recognising relationships which are full of love and full of significance".

She said she knew people would think it a "pretty eccentric view for someone like me to have because, as people know, I'm not a religious person so it's not a religious reason".