The Australian Labor Party's vote to support gay marriage brings the party in line with mainstream values but the fight for marriage equality isn't over yet, the Australian Greens say.

"Unfortunately true equality can not exist when there are caveats, while there is opt-out clauses," Greens Senator Hanson-Young told reporters in Sydney on Saturday.

"Rather than seizing the moment and bringing forward a guarantee of this legislation passing the parliament, the Labor party has opted out."

Earlier on Saturday the ALP voted in favour of same-sex marriage and backed a motion to to allow state and federal Labor MPs a conscience vote on gay marriage if a bill comes to parliament.

The motion on the conscience vote was carried 208 votes to 184.