Attorney-General Simon Corbell will announce the bill on Monday. It is set to make the ACT the first Australian jurisdiction to introduce a marriage scheme for same-sex couples. The Legislative Assembly is expected to debate and pass the same-sex marriage laws in October.

The reform was promised by ACT Labor before last year's territory election. The government, with Greens Minister Shane Rattenbury, has the numbers on the floor.

The laws will not have a residency requirement, meaning same-sex couples who do not live in the territory will be able to have a marriage ceremony in the ACT. But the legislation could be open to challenge in the High Court and Christian lobbyists have already warned they will pressure the Abbott government to override the laws in the Federal Parliament.

Mr Corbell said the government had modelled its bill on legislation to be introduced to the NSW Parliament, which is widely expected to have a vote on same-sex marriage by the end of the year. The laws will allow same-sex couples to have their marriage solemnised by an authorised marriage celebrant.

''I think we've seen that this issue is a matter that people feel very strongly about,'' Mr Corbell said.

''We've seen some attempts in other state parliaments to legislate without success.

''We know there's a majority on the floor in the Assembly who are going to support this reform.''

Mr Corbell said the government would ''robustly'' defend the laws in the event of a challenge.

''It would be interesting to see what the federal government's position is,'' he said. ''But I think the tide is changing on these matters.

''There's a clear majority of Canberrans who support this reform.

''A clear majority of Australians support this as well. And I think there is a majority across the major parties that support this reform.

''So whether an Abbott government gets involved in this debate, we'll have to wait and see.

''But I will repeat that the territory government is capable of enacting state-based law in the same way that a state government is.''

The federal government used to have the power to veto territory laws by advising the Governor-General to disallow them.

But under recent changes to the Self Government Act, it would take a vote in both houses of the Federal Parliament to overturn the laws.

Mr Corbell said there should have been reform to the national Marriage Act but ''there is a stalemate'' in the Federal Parliament.

The ACT government has a long history of advocating laws to recognise same-sex partnerships, dating back to its civil unions legislation in 2006, which was quashed by the Howard government but re-enacted last year.

''This is something that I have consistently, albeit quietly, championed,'' Mr Corbell said. ''It would be something I would be very proud of if the territory were to become the first jurisdiction in the country to legislate for same-sex marriage.''

Lyle Shelton, managing director of the Australian Christian Lobby, said at the weekend that the Federal Parliament should be prepared to override an ACT same-sex marriage law.

''It's not really appropriate for the ACT Legislative Assembly to be redefining marriage and I think it will become necessary for the Commonwealth Parliament to protect the constitution and overrule the ACT, should it go down this path of redefining marriage,'' he said.