Labor leadership contender Bill Shorten says it would be up to the caucus to decide how it votes if the Abbott government moves to overturn the ACT's same sex marriage laws.
The Labor ACT government introduced marriage equality laws on Thursday and is poised to pass them in October.
But Prime Minister Tony Abbott has already indicated he might try to get the law overturned by federal parliament, saying he's asked Attorney-General George Brandis to seek legal advice.
A territory law can be overturned by a vote of both houses of federal parliament.
Mr Shorten says he supports gay marriage but any parliamentary vote would be a matter for caucus.
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"We have to see: one, what gets passed through the ACT and then two, what will Mr Abbott do about it?" Mr Shorten told Sky News on Friday.
"This will be a matter for the caucus but I think people are entitled to know my personal beliefs.
"I did support the gay marriage legislation."
He called on Mr Abbott to let coalition MPs have a conscience vote on the matter, should it come up in federal parliament.
"Mr Abbott's going to have to see if he believes in telling his people what to do or if he's going to engage in allowing Liberal MPs freedom of thought," he said.
Former Labor minister Craig Emerson said even if Mr Abbott did try to overturn the law, he probably wouldn't get the motion through the current Labor and Greens-dominated Senate.
