The ACT is ready to fight for its same-sex marriage law in the High Court on behalf of all couples who use it to wed.
The commonwealth has already lodged a writ with the High Court to challenge the new same-sex marriage law, which only passed the ACT Legislative Assembly on Tuesday.
ACT Attorney-General Simon Corbell said the challenge was flagged a fortnight ago.
"I'm confident the ACT is well advanced in its arguments and its submissions," he told reporters in Canberra on Wednesday.
"Those arguments will also be made on behalf of all of those same-sex couples who have chosen to marry under that law."
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The case was one of "uncharted legal waters", he said.
"No one can be confident about what the High Court will decide, but I'm very confident that our Act is soundly based and has the potential to give us strong legal arguments to put to the court."
The commonwealth argues the ACT Marriage Equality (Same-Sex) Act 2013 is inconsistent with the Commonwealth Marriage Act 1961.
The Marriage Act 1961 establishes the sole and exclusive means under Australian law by which the status of marriage may be attained, it says.
It also argues the ACT law is inconsistent with the Family Law Act 1975, which makes provisions for divorce and what happens with property and children.
The ACT believes its legislation creates a separate form of marriage that can exist concurrently with federal laws.
The commonwealth wants the case heard as soon as possible to avoid uncertainty over the ACT legislation as same-sex marriages could be held as early as December.
The commonwealth is proposing the case be ready to proceed by late November.
Mr Corbell said the ACT would co-operate with any directions from the court as to timing.
But he said the Territory would urge the court to be mindful of making sure other states and parties who wished to join the action had enough time to prepare their arguments.
Lobby group Australian Marriage Equality will seek leave to join the action.
"We will also continue to work with the ACT government to ensure its law is as robust as possible, which may involve further amendments," Australian Marriage Equality national director Rodney Croome said.
The ACT expects to file its defence early next week and a directions hearing could be heard later in the week.
