Attorney-General Simon Corbell said on Thursday the government believed its laws were valid and ''attempts to stymie, block or prevent reform will only lead to an even greater impetus for it''.

The federal government will challenge the ACT's same-sex marriage laws in the High Court as soon as they are passed.

Attorney-General George Brandis described the territory laws on Thursday as ''a threat'' to the ''well-established position'' that marriage laws should be nationally consistent and were the domain of the Commonwealth.

Senator Brandis informed the ACT government of the legal challenge in a phone call on Wednesday night before confirming the move at a meeting of state and territory attorneys-general on Thursday.

The ACT government says it will ''robustly defend'' its laws, which are expected to pass at the October sitting of the ACT Legislative Assembly with the support of all eight Labor MLAs and Greens MLA Shane Rattenbury.

The federal government will ask for an expedited High Court hearing and has called on the ACT government not to pass its laws until their constitutional validity is tested. Chief Minister Katy Gallagher said on Twitter that the ACT government had declined that request.

''It would be very distressing to individuals who may enter into a ceremony of marriage under the new ACT law, and to their families, to find that their marriages were invalid,'' Senator Brandis said.

He said the advice of the acting federal Solicitor-General was that the ACT's marriage equality bill was invalid because it was inconsistent with the Commonwealth Marriage Act.

''Irrespective of anyone's views on the desirability or otherwise of same-sex marriage, it is clearly in Australia's interests that there be nationally consistent marriage laws,'' he said.

''At the moment, the Commonwealth Marriage Act provides that consistency. The ACT's proposed law is a threat to that well-established position. It has been understood for more than half a century that there is a single Commonwealth law governing marriage in Australia. The Abbott government believes that that should continue to be the case.''

Mr Corbell said on Thursday: ''I'm not surprised but I'm disappointed that there is a failure to acknowledge that this fundamental matter of inequality needs to be addressed.

''Attempts to stymie, block or prevent reform will only lead to an even greater impetus for it,'' he said.

Mr Corbell said there ''are still battles to be fought'' and reiterated the ACT government's intention to ''robustly defend our law in the court''. ''The Commonwealth have confirmed … that they will be commencing legal proceedings in the High Court once our laws become operational,'' Mr Corbell said. ''They argue it's inconsistent. I indicated the territory's view, legal advice and the advice of other constitutional law

experts differed from that and we will be robustly defend our law in the court.''

Mr Corbell said Coalition-aligned state and territory governments had indicated they would support the federal position.

He said that included NSW, whose same-sex marriage bill served as the basis for the territory's legislation.

Mr Corbell said the federal government would assert that its view of marriage covers all aspects of that matter and that it was not open to states and territories to legislate for same-sex marriage. ''We assert that our bill is capable of concurrent operation and is therefore not inconsistent.''

A Tasmanian Institute of Law Reform report on same-sex marriage, published on Thursday, concluded there was ''no legal impediment to states enacting marriage laws'' because marriage powers were shared by the states and the federal government.

The report said it was uncertain whether a High Court challenge to same-sex marriage laws would succeed.

The report's author, Amelia Higgs, said the study ''clearly dispelled the idea that the states can't do this''.

She said the outcome of a High Court challenge would come down to whether or not ''the states can enact a marriage power that doesn't clash with the Commonwealth''.

''I think what some people don't realise, or conveniently forget, is that opposite-sex marriage used to be regulated by the states,'' Miss Higgs said. ''It wouldn't be an unusual thing for the states to be the ones to get the ball rolling [for same-sex marriage]''.

Mr Corbell said the report showed that the validity of the ACT laws was ''a contested and open question''.

''There is just as much prospect that our law will be found to be valid as there is that it will be found to be invalid,'' he said.

''No one can be certain what the High Court will decide.''