Prime Minister Tony Abbott has warned anyone planning to marry under the ACT's new same-sex marriage laws to wait for the High Court to rule whether the legislation is valid or not.

The ACT yesterday became the first jurisdiction in Australia to pass laws allowing same-sex marriages.

However, the Federal Government is challenging the legislation in the High Court, arguing that it is not consistent with the Commonwealth Marriage Act.

A writ of summons was lodged with the High Court today and the matter is schedule for a directions hearing next week.

The Government says it has asked for an expedited hearing to avoid distressing any people who may marry under the laws, only to find their marriage is not legal.

"If as I think the ACT legislation turns out to be invalid under the Constitution, well then those marriages wouldn't be valid," Mr Abbott told 3AW.

"So I'd suggest to people who would like to be married under the ACT legislation - hold on 'til its validity is tested."

Mr Abbott says the Commonwealth is challenging the laws for "purely legal" reasons - and not as an argument based on morality.

"It's not a question of being for or against gay marriage," Mr Abbott said.

"It's a question of adhering to the Constitution.

"We are going to challenge this because we think that the Constitution should be adhered to."

He says there are a "range of views" in the Coalition party room on same-sex marriage and that Attorney-General George Brandis is "probably more progressive" on the issue than others.

"But the job of the Attorney is to uphold the Constitution and that's what we are determined to do, to ensure that our constitution is adhered to," he said.

Mr Abbott is personally opposed to same-sex marriage but has left the way open for the federal party room to decide if there should be a conscience vote on the issue if it comes before Parliament.

His sister Christine Forster revealed yesterday that she was engaged to her partner Virginia and wanted to get married under federal laws.

But that has not prompted Mr Abbott to change his mind.

"She chews my ear up hill and down dale on this subject," he said.

"I wish her and Virginia all the best for their future happiness and if there's a ceremony of some kind, yes I'll be there with a present. I'll do the right thing.

"But look, I am a traditionalist on this.

"From time immemorial, in every culture that's been known, marriage ... has been between a man and a woman."

In September 2012, laws legalising same-sex marriage failed to pass Parliament.

Labor MPs were allowed a conscience vote but Coalition MPs were not.