Former prime minister Kevin Rudd insists his change of heart to support gay marriage is about his "personal journey" and not a political point-scoring exercise.
Mr Rudd posted a blog on Monday declaring he had changed his mind on the controversial issue and now agreed that same-sex couples should be allowed to wed so long as churches and religious institutions don't have to marry them.
But the opposition has questioned Mr Rudd's motivations.
Previously, Mr Rudd had been a staunch opponent to gay marriage and as recently as September 2012 voted against legalising same-sex marriage in a parliamentary vote which lost 98-42.
But he insists he has had a change of heart and wanted to let the Australian public know his new stance before a new vote in the lower house on June 6.
"If you can't be grown up enough in the Australian national political debate ... and reach an amended or changed position, then frankly you shouldn't be in national political life," Mr Rudd told reporters in Brisbane on Tuesday.
"This has been something of a personal journey for me."
Mr Rudd said he wasn't concerned his new position would put him at odds with Prime Minister Julia Gillard, who has previously voted against the change.
Deputy Prime Minister Wayne Swan said he respected Mr Rudd's decision and urged Opposition Leader Tony Abbott to allow coalition MPs a conscience vote on the issue.
"This is an issue where views are deeply held and that's why it's appropriate that we have a conscience vote in the parliament," Mr Swan told reporters in Adelaide.
Foreign Minister Bob Carr says the parliamentary numbers may be shifting in support of legalising same-sex marriage.
Liberal frontbencher George Brandis said Mr Rudd's comments were more about personal ambitions than about the issue of same-sex marriage.
"What it tells you is that Kevin Rudd has not given up, Kevin Rudd is at it again," Senator Brandis told Sky News.
The Australian Christian Lobby says Mr Rudd's announcement was a huge disappointment for Christians.
"If this is an attempt to wedge Julia Gillard, it will cost Mr Rudd the last of his following in the Christian constituency," spokesman Lyle Shelton said in a statement.
Labor backbencher Stephen Jones, one of the key figures behind the initial gay marriage bill, said Mr Rudd had followed a path taken by many MPs who had begun instinctively with scepticism or opposition.
"At some stage over the next five years, we will have a change of law in our country," Mr Jones said.
The House of Representatives will revisit the issue on June 6, with a vote expected on a Greens private members' bill.
Prime Minister Julia Gillard challenged Mr Abbott to "follow in the footsteps" of conservative leaders such as John Key in New Zealand and British Prime Minister David Cameron and allow his MPs to have a conscience vote on gay marriage.
"I've made my stance clear," Ms Gillard told reporters in Melbourne, reiterating her opposition to changing the law.
"The important thing for me is that members of my political party can exercise their conscience vote."
She challenged the coalition to allow a conscience vote.
Mr Abbott said he respected Mr Rudd and the former Labor leader was entitled to change his mind.
"I certainly haven't changed my mind," he told reporters in Brisbane on Tuesday.
He reiterated the coalition's position of support for the status quo.
Labor Senator Stephen Conroy said Mr Rudd's announcement had not changed his own opposition to same sex marriage.
"We welcome this debate and individuals should have their view on this," he told reporters in Sydney.
"I voted against it in the parliament.
"I considered all the matters and in the end I ended up leaning towards 'no'.
"I've been a strong supporter of civil unions for a long time, but I don't support that jump across."