THE High Court in Australia has ruled that the Australian Capital Territory's legislation for same-sex marriage - the first in the country - is invalid.

The court's unanimous decision at midday on Thursday means that the marriages of the 27 same-sex couples who married after the legislation came into force last Saturday are also invalid.

The court found that, under the Australian constitution, only the Federal Parliament had the power to legislate for marriage, and that the current federal marriage legislation was confined to marriage between a man and a woman.

The appeal to the High Court was brought by the Coalition Federal Government.

Responding to the High Court decision, the ACT Chief Minister, Ms Katy Gallagher, said marriage equality was now "firmly at the feet" of the Prime Minister, Tony Abbott. The Greens Party Senator Sarah Hanson-Young called for marriage equality to be legislated at the federal level. "Australia, as a nation, is ready for this, and it's time the Federal Parliament recognised that," she said.