The federal government plans to challenge ACT same-sex marriage laws in the High Court when the territory enacts legislation.
Commonwealth Attorney-General George Brandis told Thursday's meeting of the standing council on law and justice that the government will start legal proceedings at once, his ACT counterpart told AAP.
"The Commonwealth Attorney has confirmed that they will be launching legal action," ACT Attorney-General Simon Corbell said shortly after the meeting in Sydney.
"They will be doing so on the basis that they argue the Commonwealth Marriage Act covers the field and therefore the ACT law is inconsistent.
"There's been a polite but forthright discussion today between myself and the Commonwealth Attorney on the matter."
The ACT Labor government expects the law to pass with the support of Greens MLA Shane Rattenbury when the ACT Legislative Assembly sits later this month.
If so, the first same-sex marriages could happen as early as December.
Senator Brandis has asked the ACT to defer the start of its law until after the High Court has ruled on the matter.
"We have declined to do that because we know there is very strong support for this law to come into effect," Mr Corbell said.
"There are many same-sex couples both in the ACT and in other parts of the country who support the law and who want ... to have the opportunity to be married, even though they understand that the Commonwealth has announced that it will challenge the law."
If the law is struck down, any same-sex marriages would become null and void, although Mr Corbell said the ACT would recognise those commitments in another way, such as civil unions.