Canberra - A provincial parliament on Tuesday passed the first law allowing same-sex marriage in Australia, but it could be short-lived with the federal government threatening a court challenge.
The Australian Capital Territory parliament passed the law with the support of lawmakers from the province's governing Labor and Greens parties. All eight opposition Liberal Party lawmakers in the 17-seat Legislative Assembly voted against the bill.
The first marriages could take place in the national capital Canberra in December, but federal Attorney General George Brandis, a member of the Liberal Party, said his government has legal advice that the legislation is invalid.
Australian Capital Territory Chief Minister Katy Gallagher refused a request from Brandis to wait on allowing any same-sex marriages until the High Court ruled on the law's constitutional validity.
Australian federal law was amended in 2004 to specify that marriage can only be between a man and a woman. But it also specifically applied to heterosexual couples, and some lawyers argue that leaves states free to legislate for same-sex marriage.
Prime Minister Tony Abbott opposes gay marriage and his coalition last year thwarted federal bills that would have allowed legal recognition of same-sex relationships.
His sister Christine Forster disclosed on Nine Network national television on Tuesday that she is engaged to her partner of six years, Virginia Edwards. Forster said they would not marry until they could do so in their hometown of Sydney.
Forster said the prime minister supported her relationship. However brother and sister disagreed on whether same-sex relationships should be legally recognised as marriages.
"He's always said: 'Well, I'll be there at the wedding, Chris,'" Forster told Nine.
Gay lobby group Australian Marriage Equality said thousands of same-sex couples from across Australia had shown interest in tying the knot in Canberra.
The capital territory encompasses mainly Canberra, with a population of 360 000.
Constitutional lawyer George Williams, of the University of New South Wales, said there were no precedents to suggest which government would likely win the High Court case. "It's a genuinely open question," said.
Federal Parliament could also pass legislation to remove the Australian Capital Territory law. But the government could not rely on the Senate backing such federal intervention.
"They may have reached an assessment that that may not be easy at the moment and also perhaps they don't want that sort of debate" the federal parliament, Williams said.