Solicitor-General Peter Garrisson told a Legislative Assembly hearing on Monday that the ACT had paid $500,000 in legal costs to the Commonwealth Government after the nation's first law allowing couples of the same sex to marry was overturned in December 2013.

Bills for two external lawyers engaged for the case totalled $112,000 and Mr Garrisson said the cost of work completed by government lawyers was about $200,000.

In December last year the High Court found the law, passed by the ACT Assembly in October 2013, could not operate alongside the Commonwealth Marriage Act as only the federal Parliament had the power under the Constitution to legislate on marriage.

The court held the ACT's law provided for marriage equality for same-sex couples, not, as the territory argued, for a form of legally recognised relationship that was different from marriage.

The finding meant the ACT law could not operate concurrently with the federal act. The territory was ordered to pay the Commonwealth's legal costs.

Just 31 same-sex couples had married before the law was overturned.

Giving evidence to a hearing on ACT Government annual reports, Mr Garrisson said he would not discuss the legal advice he provided to the government.

There were "unknown unknowns" in the case, he told Opposition Leader Jeremy Hanson, an opponent of same-sex equality.

"I believe, presented with the government's initiative and the advice that I provided with other Senior Counsel to government, that we would do very little different," Mr Garrisson said.

"The issues, whilst dealt with in short compass by the High Court, addressed some pretty fundamental issues that to that point had not been resolved. The nature and scope, for example, of the marriage power, the nature and scope of the Commonwealth Marriage Act, had not been canvassed in this environment."

Attorney-General Simon Corbell told the hearing costs associated with case, including the engagement of external lawyers, had been met within the ACT Government Solicitor's existing budget.

"To put the costs in some context, in relation to external counsel, the territory expends approximately $2.5 million every year on average for a variety of legal cases ranging from the Civil and Administrative Tribunal, the Magistrates Court, Supreme Court, the Court of Appeal and the High Court from time to time," Mr Corbell said.

In a statement after the hearing, Mr Corbell said he "makes no apology for defending the principle of marriage equality in the High Court.

The legislation was an election commitment from ACT Labor in 2012 and was passed with the support of Greens minister Shane Rattenbury.

A recent poll commissioned by Australian Marriage Equality found 72 per cent of Australians want same-sex marriage legalised and 77 per cent think federal Coalition MPs should be granted a conscience vote.

The poll found opposition to same-sex marriage has all but collapsed as a political issue, with just 21 per cent opposed to the reform.

Labor frontbencher Tanya Plibersek and Liberal Democratic Senator David Leyonhjelm have both proposed new bills on same-sex marriage. Prime Minister Tony Abbott has said any changes to marriage laws will be a matter for the Coalition party room.