Labor MPs and senators will not be bound to vote for same-sex marriage until 2019.

After hours of negotiations, the party's Right faction cobbled together the numbers to amend a motion from the Left that would have forced MPs and senators to be bound in the next term of parliament.

The revised motion, successfully moved by Labor leader Bill Shorten, ensures members will be given a conscience vote on the issue for another two terms of government.

"What the Labor Party does with this resolution is we lay down the challenge to Mr Abbott and his Liberals — please give your members of parliament a free vote so we can make marriage equality a reality now," he said.

As part of the compromise deal, Mr Shorten pledged to move to legalise same sex marriage should he win the election.

"I promise that within 100 days of a Labor government being elected that I shall move in the parliament of Australia for marriage equality for Australians," he said.

"Marriage equality is a simple, overdue change that sends a powerful message."

Deputy leader Tanya Plibersek had been pushing for a binding vote but seconded Mr Shorten's motion at the conference.

"I still hope we can have marriage equality by Christmas, but if this Parliament doesn't pass marriage equality a Shorten-Labor government will within its first 100 days," she said.

"I know there are people in the room who feel they have waited long enough and beyond long enough to have the issue of marriage equality taken up by the whole of the Labor Party as an issue about legal discrimination."

Labor senator Penny Wong is openly gay and has long been pushing for a binding vote to legalise same sex marriage.

"I commend a resolution which ends the conscience vote in the Labor Party on a matter that should never have been a conscience vote," she said.

Just hours before the debate, left-wing delegates were confident their motion to bind members on a same-sex marriage vote would be passed at the national conference.

However, Mr Shorten managed to head off the eleventh hour push, in what is being seen as an important win for the Labor leader.