Prime Minister Tony Abbott says challenging the ACT's same-sex marriage laws in the High Court is not a moral issue but a constitutional matter.
Federal Attorney-General George Brandis has instructed commonwealth lawyers to challenge the laws which passed the ACT Legislative Assembly on Tuesday.
Mr Abbott said the Australian constitution clearly stated that the federal government is responsible for the regulation of marriage.
"It is not a question of being for or against gay marriage - it's a question of adhering to the constitution," Mr Abbott told Fairfax Radio on Wednesday.
He said he would advise any gay couples thinking of using the ACT legislation in the next few months not to do so.
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"If, as I think, the ACT legislation turns out to be invalid under the constitution then those marriages wouldn't be valid," he said.
"So I would suggest to people who would like to be married under the ACT legislation hold on until its validity is tested."
He said there was a range of views about same-sex marriage in the coalition and within the cabinet itself.
"I suspect the attorney's view is probably more progressive than some others' views might be," Mr Abbott said.
"But the job of the attorney is to uphold the constitution."
The prime minister's gay sister Christine Forster has said she wants to marry her partner in NSW.
Mr Abbott said he wished his sister the best for the future, and would attend her ceremony if it occurred, but his "traditionalist" view remained.
"From time immemorial, in every culture that has been known, marriage or that kind of solemnised relationship has been between a man and a woman."
