Darlene Cox and partner Liz Holcombe at their home in Downer.

Darlene Cox and partner Liz Holcombe at their home in Downer. Photo: Melissa Adams

Darlene Cox has already made history once and could do it again when same-sex marriage is legalised in the ACT.

The Downer woman has been with partner Liz Holcombe since they met at work 15 years ago.

''It was one of those whirlwind romances,'' she said.

''We met each other and instantly knew we wanted to spend the rest of our lives together.''

Ms Cox and her partner were the first female couple to register for a civil partnership in the capital, among the 226 couples that signed up when the first legislative changes were introduced in 2009.

Four years later, they're ready to become one of the first same-sex couples to wed in Australia as the ACT government prepares to table new marriage legislation.

The government will introduce the Marriage Equality Bill 2013 to the Legislative Assembly on Thursday.

The bill will be introduced next month and is likely to pass with the support of all eight Labor MLAs and Greens member Shane Rattenbury.

An ACT marriage equality law could face a challenge in the High Court, but ACT Attorney-General Simon Corbell told ABC TV on Tuesday he expected the bill to be operational before the end of the year and that it would operate concurrently to federal legislation.

''What's very clear is that the Commonwealth Marriage Act is an exclusive scheme that applies only to heterosexual couples,'' Mr Corbell said.

''Now under our constitutional arrangements, states and territories are able to legislate for relationships other than relationships as set out in the Marriage Act.

''We assert that there is clear grounds for the territory to both legislate on this question and for the act, should it be adopted by the Legislative Assembly, to be capable of concurrent operation consistent with the relevant provisions of the ACT Self-Government Act.''

But the ACT Liberals argue the assembly should refrain from debating the issue because marriage laws are a matter for the Federal Parliament.

Ms Cox stressed that getting married was a normal part of life to which everyone should have access. ''I think it's great we have a progressive government. There will be other parts of the community who see this as a threat to the institution of marriage as they know it.''

While there are numerous options for same-sex marriage beyond Australia's borders, Ms Cox said she and Ms Holcombe wanted to get married in Canberra.

''We love this city and we want to get married at home,'' she said.

Some supporters of traditional marriage have urged the Federal Parliament to override any attempt by the ACT to introduce a local same-sex marriage scheme.