An upcoming gay marriage law in Australia could be knocked down just days after taking effect on Saturday.

The Australian Capital Territory (ACT) approved the law allowing gay and lesbian couples to marry in the territory, which includes Canberra, the nation's capital, in October.

Prime Minister Tony Abbott, whose conservative Liberal-National coalition swept into power on September 7 on a pledge to lower taxes, immediately filed a legal challenge to the law, arguing that only the federal government has the power to regulate marriage.

According to the AP, the court heard arguments in the case on Tuesday and announced that it would deliver its ruling on December 12.

Lawyers representing the ACT government argued that the law was legal because it only applies to couples that do not qualify under the federal government's definition as being a heterosexual union.

Abbott, who has repeatedly stated his opposition to marriage equality, warned gay couples from marrying in the ACT until after the High Court has ruled on the legislation's validity.

“If as I think the ACT legislation turns out to be invalid under the constitution, well then those marriages wouldn't be valid,” 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.”

However, at least a dozen gay couples are preparing to inaugurate the law on Saturday.