NSW Labor has voted to change its rules to support gay marriage.

But to cries of “boo” and “shame” from socialist left delegates, the party’s state conference rejected a motion that would’ve called on federal MPs to be forced to vote on party lines in favour of same-sex marriage.

Despite federal Labor supporting gay marriage, its MPs are free to vote according to their personal views in parliament.

The motion, which was rejected 352 to 284 votes on Sunday, would’ve called on the next federal national ALP conference to amend the party’s platform so that Labor parliamentarians be denied a conscience vote on the issue.

The conference ballot was a closer contest than many were expecting.

Supporters of gay marriage told the conference, held in Sydney’s Town Hall, the matter was not one of conscience but of human rights.

But Senator Sam Dastyari said while he supported gay marriage, he was against forcing others in the party to vote with him.

He said it was hypocritical for the party to push the Liberal Party, who oppose gay marriage, to allow a conscience vote on the issue while calling for the same right to be taken away from Labor MPs.