Prime Minister Tony Abbott says same-sex couples who married in the Australian Capital Territory (ACT) knew there was a possibility their marriages would be annulled.

On Thursday, the High Court unanimously ruled that the ACT’s laws were inconsistent with the Federal Marriage Act and were therefore unconstitutional.

The court said the issue should be decided by the Australian Federal Parliament – which in September 2012 voted down equal marriage legislation.

Some 27 couples who married since the law came into effect last weekend will now have their unions declared invalid.

The ACT Parliament passed a bill in October making the territory the first part of Australia to legalise equal marriage.

But the Federal Government – under the instruction of Prime Minister Tony Abbott – challenged the decision, saying it was inconsistent with federal laws.

In October, Mr Abbott warned same-sex couples against marrying in the ACT until legal proceedings had concluded.  

On Friday, Mr Abbott said his position on the issue had not changed and that he remains opposed to equal marriage.

He said the High Court decision was obviously disappointing for the same-sex couples who had recently married in the ACT.

“They knew that there was this possibility that their marriages might fall foul of the High Court and obviously it’s disappointing for them,” he said.

“Let’s see what the future holds.”

Mr Abbott said if a bill to legalise same-sex marriage comes before the Australian Federal Parliament, it will be dealt with in the usual way.

“You know my position, I don’t think that it should change,” he said.

“I suspect even up here amongst my fellow first ministers there’ll be a range of views, if we have a bill come before the Parliament if will be dealt with by the Coalition in the usual way.”

Mr Abbott is facing fresh calls from senior members of his Liberal Party to allow MPs a free vote on legalising same-sex marriage.

Federal Communications Minister Malcolm Turnbull, a strong advocate for equal marriage, said it was significant that the court ruled same-sex marriages were contained within the context of marriage in the Constitution, but it was up to the Federal Government to legislate on the matter.

“I think there is a reasonable prospect of a change to the law in this parliament,” Mr Turnbull told ABC radio on Friday.

He believes it could come in the form of a Private Member’s Bill.

Christine Forster, the gay sister of Mr Abbott, has repeatedly called on her brother support equal marriage. 

Australian federal law was amended in 2004 to specify that marriage can only be between a man and a woman. Equal marriage remains banned at a federal level in Australia.