CANBERRA, Australia (AP) — A provincial Australian parliament began debating legislation on Tuesday that would allow same-sex marriages despite the threat of a court challenge by the federal government.

The Australian Capital Territory parliament began debating the bill that would create Australia’s only law allowing gay couples to marry. It was likely to be passed later Tuesday with the support of lawmakers from the province’s governing party, despite all eight opposition lawmakers in the 17-seat Legislative Assembly announcing they would vote against the bill.

A hand-made placard is held up during a gay rights demonstration in Sydney in 2011. Pic: AP.

Federal Attorney General George Brandis has threatened to challenge the validity of the law in the High Court if the bill becomes law and allows same-sex marriage in the national capital Canberra.

Australian federal law was amended in 2004 to specify that marriage can only be between a man and a woman.

Prime Minister Tony Abbott opposes gay marriage and his coalition has 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 gay partner of six years, Virginia Edwards.

Forster said the couple would not marry until they were able to do so in their hometown of Sydney, either because of a change to federal or New South Wales state law.

Forster said the prime minister supported her relationship. However brother and sister disagreed on whether same-sex relationships should be legally recognized 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.

If the legislation is passed, the first same-sex marriage ceremonies could be held in Canberra from December.

Australian Capital Territory Chief Minister Katy Gallagher said she had refused a request from the federal government not to allow same-sex marriages to take place until the High Court ruled on the law’s constitutional validity.

“I think this is an issue that captured the imagination of people right around the country,” Gallagher told Australian Broadcasting Corp. radio. “The ACT has never been afraid to lead the way.”