State and territory governments have no right to legislate in favour of same-sex marriage in defiance of commonwealth law and the constitution, the federal government says.
Prime Minister Tony Abbott made the stand on Friday, a day after Attorney-General George Brandis warned the validity of a proposed ACT law allowing same-sex marriages would be challenged in the High Court.
Senator Brandis also personally asked ACT Chief Minister Katy Gallagher not to allow same-sex marriages to proceed if the bill was passed during the late October sitting of the ACT Legislative Assembly.
But the ACT says it won't be bullied by the federal government and will push ahead with the bill, which could see the territory hosting same-sex marriage ceremonies by December.
Asked what message the High Court challenge would send to gay and lesbian couples wanting to get married, Mr Abbott said: "We want to uphold the constitution."
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"We think it is important that there be a uniform approach to marriage around the commonwealth, and that's what we are going to do our best to ensure," he told reporters in Darwin.
"It's important to ensure that the constitution is adhered to."
Federal parliament last year rejected a Labor private member's bill aimed at altering the definition of marriage, as being between a man and a woman, to allow same-sex marriage.
Mr Abbott said while future federal parliaments may revisit the issue, the bill was defeated by a "fairly decisive margin".
"The point is that it's the commonwealth parliament which is responsible for making laws with respect to marriage, not the state parliaments, and not the territory legislatures," he said.
Labor federal leadership candidate Bill Shorten said he supported the principle of states and territories having the right to make their own laws.
"It's a matter of record that I voted for same-sex marriage when the matter came before the federal parliament, but I don't think this issue is just about that," he told ABC Television.
"You've also got some pretty important constitutional issues."
Australian Greens leader Christine Milne said the looming High Court challenge was a front for the coalition government's opposition to marriage equality and reform.
"If they were genuinely interested in taking away any question of whether the commonwealth should only have the power to legislate with regard to marriage, then we would simply be passing the legislation in the federal parliament," Senator Milne said.
Lobby group Australian Marriage Equality (AME) said the federal government's reasoning wasn't clear.
"To say it is about the constitution is not a clear enough answer," AME chair Alex Greenwich said.
"Constitutional experts, the Tasmanian Law Reform Institute and the NSW parliamentary inquiry all found that it is constitutionally possible for a state to legislate for same sex marriage."
The ACT argues that because the commonwealth marriage act applies to unions between men and women, it's within its right to legislate beyond that to same-sex couples.
