NSW Labor is set to back same-sex marriage in Australia at next weekend's annual conference, three years after dodging the issue in 2011 by referring it to the federal division.

But the party is expected to be split over whether to call on the federal Labor party to require its MPs to vote in favour of same-sex marriage legislation that may come before the House of Representatives.

A motion to support same-sex marriage is expected to be backed by state Labor's annual conference on July 26-27, with Right faction delegates to be allowed a conscience vote.

The Left will vote in favour of the move, virtually guaranteeing the motion will pass.

However, the fate of a separate - and potentially more significant - motion calling on the party's national conference to force federal MPs to vote in favour of marriage equality when legislation next comes to the parliament is far less certain.

At present federal Labor MPs are granted a conscience vote on same-sex marriage. But supporters argue it is less of a social issue - for which conscience votes are usually granted - and more of a human rights matter on which the party's MPs should be directed.

In September 2012 the House of Representatives voted down a bill that would have legalised same-sex marriage in Australia by 98 votes to 42. Then Prime Minister Julia Gillard supported Labor MPs having a conscience vote.

The motion at next weekend's state Labor conference will call on the federal conference to require Labor MPs to bind in support of same-sex marriage legislation.

If the motion passes and national conference was to adopt the recommendation, having all Labor MPs voting to support same-sex marriage would improve the chances of federal legislation being passed, but only if the Coalition is granted a conscience vote.

Prime Minister Tony Abbott has so far resisted granting Liberal MPs a conscience vote on the issue and the Coalition policy is to opposes same sex marriage.

On Monday, Liberal Democratic Party Senator David Leyonhjelm said he will introduce a private members bill into the Federal Parliament later this year to legalise same-sex marriage once the Coalition agrees to a conscience vote.

The move to support same-sex marriage at next weekend's conference will bring the NSW Labor party into line with every other state Labor branch and the national conference.

Opposition Leader John Robertson said he is a "proud father of a gay son" and was "pleased it's on the conference agenda".

"I respect that others have deeply held views on this and strongly believe we should continue to allow people to vote according to their conscience," he said.