Victorian Labor Party had been set to debate the motion on Saturday but its conference came to an abrupt end when the meeting was deemed to be three people short of the required numbers.
The state party had already backed same-sex marriage at its 2009 conference but the latest move would have gone a step further, calling on the ALP national conference in December to incorporate support for gay marriage in the party's policy position.
Hundreds of Labor faithful packed a Monash University hall to hear Prime Minister Julia Gillard, who opposes same-sex marriage, and state Labor leader Daniel Andrews, who supports it, talk about the state of the party.
But numbers had dwindled by Saturday afternoon and debate on the marriage equality issue and more than 20 other urgency motions never went ahead because there were only 147 delegates in the hall when 150 were required for a quorum.
Secretary of the ALP's policy committee on gay and lesbian issues, Jamie Gardiner, said the developments did not change the fact that the Victorian branch had already backed marriage equality.
"Today's motion was simply to reiterate that support and to echo the South Australian branch's resolution in support, calling on the national conference to support marriage equality," Mr Gardiner said.
"I can only assume that elements in the party who knew they would lose engineered a walkout to bring the conference to a premature close."
Other motions had supported same-sex adoption and condemned Ms Gillard for her asylum-seeker deal with Malaysia.
ALP state secretary Noah Carroll said the Victorian branch backed Ms Gillard.
"The prime minister is her own person, everyone understands that, but she has the full support of the party in Victoria and presumably also the federal caucus."
In her first address as prime minister to a Labor Party conference in her home state, Ms Gillard used her keynote speech to push the case for a carbon tax, but left before the urgency motions were to be debated.
She said was proud of Labor's record in Victoria, where it was in power for 11 years before November's election defeat.
We've seen a premier paralysed by indecision, frozen solid, unable to make the hard decisions that Victoria and Victorians need," the opposition leader said.
Internal ructions over the Broadmeadows by-election in February again resurfaced with a motion put to the conference criticising the way Frank McGuire was chosen and calling for changes to the pre-selection process.
Mr McGuire, who ultimately retained the seat vacated by former premier John Brumby for Labor, was selected by the party's national executive after a bitter internal factional brawl and a bloc of unions unsuccessfully challenged his pre-selection in court.
The motion, however, was referred to the party's rules committee for further debate at the October state conference.
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