Australian backers of gay marriage believe their cause will be boosted by the United States President's change of heart on the issue but concede they do not yet have enough support in the federal Parliament to have the Marriage Act amended.

Mr Obama used a television to interview yesterday to announce that he had come to the conclusion that same-sex weddings should be permitted. ''I've just concluded that for me, personally, it is important for me to go ahead and affirm that I think same-sex couples should be able to get married,'' he said.

Ms Gillard said she would vote against a private member's bill to amend the Marriage Act when it came before Parliament later this year.

Pressed on the issue by ABC Melbourne radio host Jon Faine, Ms Gillard said: ''I believe what I believe Jon, you can judge it.''

The ACT's Deputy Chief Minister Andrew Barr, who is openly gay and last year was successful in having gay marriage placed in Labor's national platform, conceded President Obama's comments would now put pressure on Ms Gillard to present a similar view.

''Yes - I suspect it does,'' Mr Barr said. ''To her credit, the Prime Minister allowed a genuine debate on the issue at Labor's national conference last year. It was a great honour to successfully move the historic marriage equality platform amendment. I believe marriage equality in Australia is inevitable and look forward to that change being led by a Labor prime minister.''

Mr Barr said he followed President Obama's Twitter feed, so found out about the news there, adding he felt ''very happy - like getting married one day''.

There was no underestimating the importance of the President's support for gay marriage. ''The US President is hugely influential in world political circles. His announcement may well be a turning point in this debate,'' Mr Barr said. Mr Barr and his partner, Anthony Toms, celebrated their civil partnership in November 2009 in a ceremony at the National Library.

Veteran gay rights activist Rodney Croome said the chances of a gay marriage bill passing Parliament would be boosted if the Liberal Party followed the example of Labor and formally granted its MPs a conscience vote on the issue.

''While I wouldn't predict that marriage equality would get up with a bipartisan conscience vote, I think the numbers would be close,'' he said.

Mr Croome believed it was inevitable that Australia's marriage laws would eventually be amended to recognise same-sex relationships.

In the United States, marriage laws are a matter for individual states. In Australia, marriage is regulated by the federal Parliament.

Australian Christian Lobby chief Jim Wallace did not believe Mr Obama's comments would have much influence on debate in Australia. ''Obviously it's Obama's opinion, but even the opinion of the President of the United States doesn't change biology or biological truth.

Biological truth is that marriage is between a man and a woman,'' he said.