Attorney-General Simon Corbell introducing the Marriage Equality Bill in the ACT Legislative of Assembly.

Attorney-General Simon Corbell introducing the Marriage Equality Bill in the ACT Legislative of Assembly. Photo: Rohan Thomson

The fate of the ACT's same-sex marriage bill could become clear as early as Thursday.

Attorney-General Simon Corbell will present information about the bill to a two-day meeting of federal, state and territory attorneys-general, amid speculation the federal government is about to challenge it.

''I will be placing the matter on the agenda and explaining why we believe the bill is valid,'' he said. ''I am not asking for any explicit agreement … I'm hopeful there will be an opportunity for an exchange of views to get a better understanding of the Commonwealth view on the matter.''

Mr Corbell said the ACT bill could operate concurrently with the federal legislation.

''Our argument has been consistent both in relation to civil unions and same-sex marriage that the Legislative Assembly can legislate for matters other than marriage as defined under the [federal] Marriage Act,'' he said.

Federal Attorney-General George Brandis has already received legal advice the ACT's gay marriage laws are unconstitutional, according to one media report.

His office did not respond on the issue on Tuesday.

Prime Minister Tony Abbott is considering whether the proposed law should be blocked as the Commonwealth has constitutional authority for marriage.

The Abbott government could challenge the ACT laws in the High Court or attempt to disallow it through votes in both houses of Parliament.

The ACT government's bill to permit same-sex marriage is expected to pass the 17-member Legislative Assembly this month with the support of all eight Labor members and Greens minister Shane Rattenbury.

The ACT government believes its bill could operate alongside the federal Marriage Act because the territory law would only apply to same-sex couples, not heterosexual relationships.

Mr Corbell said Senator Brandis would chair the meeting, the first gathering of attorneys-general since the federal election.

''My paper will simply outline what the bill is, what it does and that we believe it is valid,'' he said.

''It's not asking for a decision of the meeting except to note the paper.''

Mr Corbell said it was pre-emptive to be discussing High Court action.

''It still obviously hasn't been passed yet,'' he said. ''I wouldn't expect to see the [federal] legal advice … hopefully it will be an opportunity to get a bit more insight on the matter.

''The [ACT] bill is about recognising relationships other than those defined under the Commonwealth Marriage Act.

''Our argument is that our law is capable of concurrent operation because the Commonwealth Marriage Act is an exclusive scheme only for heterosexual couples and therefore schemes devised by the state and territories to recognise same-sex relationships are capable of concurrent operation alongside the Commonwealth Marriage Act.''

Former ACT chief minister Gary Humphries is questioning whether the ACT government can claim jurisdiction for same-sex marriage.

He said there appeared to be a reversal of the previous position taken by the territory.

Mr Humphries, who recently retired as Liberal senator for the ACT, said he did not believe there was a case for same-sex marriage, other than symbolism.

''I have doubts about whether the Legislative Assembly does have the capacity to pass legislation enacting same-sex marriage given that the argument in 2006 when the Assembly tried to do civil unions was that they had responsibility for arrangements other than marriage and the Commonwealth Parliament had responsibility for marriage,'' he said.

''So it seems to me that there's now a reversal of that situation and I'm not clear why that position seems to have changed, from the point of view of the Labor Party in the Assembly.''

In 2006, the Howard government used a ministerial order to rescind gay marriage laws in the ACT.

Mr Abbott would need majority votes in both houses of Parliament to veto the ACT bill after changes to the territories self-government law introduced in 2011 by then Greens leader Bob Brown.

The ALP policy allows a conscience vote on same-sex marriage, which would likely mean right-wing Labor senators would vote with the Coalition against the ACT bill.