THE Government will struggle to get the numbers to pass gay marriage laws after the ALP national conference today altered the party's platform to make legislation possible.

A small group who heckled speakers favoring same sex unions showed the task could be difficult.

A private member's bill - which would not be formally caked by the Government - is expected next year.

The key question now is what the Opposition will do. Three weeks ago Opposition Leader Tony Abbott said there would be no conscience vote but yesterday was equivocal.

Mr Abbot could see some of his MPs defy his orders and cross the floor or abstain.

At the ALP conference, rebels wrecked a strategy for Prime Minister Julia Gillard's motion granting a conscience vote to be passed on the voices.

They forced a count of the 400 delegates. The amendment was adopted by 208 votes to 184.

But the central motion, amendment 101 which would allow same sex marriage laws, was passed on cheers of support and no formal count was held.

Its backers said it was a victory for traditional Labor values of equality and human rights. However, not all members shared that view.

A Tasmanian senator, Helen Polley, said she had been advised by unnamed colleagues to not turn up today because she opposed same sex marriage. But Senator Polley acknowledged she was in a minority and had been unfairly labeled "a homophobic".

From 10 to 12, Labor MPs could vote against same sex marriage legislation in Parliament. If the Opposition decides to vote against gay marriage legislation it would not get through the House of Representatives.

Party figures today warned a change in marriage laws could cost vote in Queensland, and in seats with many retirees such as on the NSW central coast, and seats with a significant number of religious and conservative Moslems and Roman Catholics such as in western Sydney.

However, it could bolster Labor in its battle against the Greens in inner-city areas.

Conference gave MPs a free parliamentary vote on the issue, allowing them to decide according to their conscience and not by the direction of party leaders.

Prime Minister Julia Gillard successfully moved the conscience vote motion, telling the conference at Sydney's Convention Centre Labor had treated marriage issues, such as no-fault divorce, as issues of conscience.

However Senator John Faulkner said there could be no equivocation about a human rights matter: "A conscience vote on human rights is unconscionable," said Senator John Faulkner."

The delicate balance between changing the platform to insist Labor back marriage equality, but allowing MPs the the right to not support it, was negotiated to protect Julia Gillard.

The Prime Minister did not want same sex marriage to descend into a nasty internal party brawl, and did not want to publicly back gay marriage.

Senator Penny Wong, who moved amendment 101, thanked Julia Gillard and said it was "a messages of the woman" she encouraged the change.