Australia’s High Court has struck down a law passed in the Australian Capital Territory permitting same-sex marriage.

The court, the highest in Australia’s justice system, held that only the federal parliament could amend the nation’s marriage laws.

The unanimous ruling found that the Marriage Equality (Same Sex) Act 2013 passed in the Australian Capital Territory (ACT) in October could not go into effect concurrently with the Marriage Act 1961 – a federal law which says marriage is between a man and a woman.

ACT is a small, self-governing territory in southeast Australia which includes the country’s capital city, Canberra.

About 30 couples have taken advantage of ACT’s law, enacted in November. Their unions will be considered invalid, as the entire legislation was struck down.

The federal government challenged the law, saying it was inconsistent with Australian law.

Pro-marriage groups said the court’s decision saying the court’s ruling had effectively put the issue to rest, Australian news website, news.com.au reports.

Australian Christian Lobby managing director Lyle Shelton said there had been 10 attempts in the past three years at state and federal levels to legislate a new definition of marriage.

“I think it’s been given a fair go. I think it’s time to move on,” he said.

Attorney-General George Brandis said the government was pleased with the decision.

It was important for people to respect the High Court’s authority, he said.

“Even if the decision had gone the other way … it’s imperative that all members of parliament, irrespective of their personal opinions, uphold and respect … decisions of the court,” Senator Brandis said.

Shadow attorney-general Mark Dreyfus said the decision confirmed Labour’s position that parliament was the appropriate place for debate on same-sex marriage.

Earlier this month, the majority of Croatians voted to outlaw same-sex marriage in a referendum held by the newest European Union member state.