Same-sex marriage laws overturned by High Court

The court handed down its findings in the landmark case about 12.15pm, although a statement announcing the decision was accidentally published on the court's website about 20 minutes earlier.

The High Court delivered its findings to a silent courtroom. A few couples sobbed silently as Chief Justice Robert French read the unanimous finding that the ACT law was entirely inconsistent with federal marriage legislation.

Couples embraced as they left the courtroom, saying to each other "it's not over".

The judges found the ACT law could not operate alongside the federal law as only the Federal Parliament had the power under the constitution to legislate on same sex marriage.

They said the federal Marriage Act did not allow or recognise marriages between same sex couples.

The court held the ACT's act provided for marriage equality for same sex couples, not, as the territory argued, for a form of legally recognised relationship that was different from marriage.

The finding means the ACT act cannot operate concurrently with the federal act.

"Because the ACT act does not validly provide for the formation of same sex marriages, its provisions about the rights of parties to such marriages and the dissolution of such marriages cannot have separate operation and are also of no effect," the statement said.

"The court held that the whole of the ACT act is of no effect."

ACT Chief Minister Katy Gallagher says marriage equality is now "firmly at the feet" of Prime Minister Tony Abbott and every federal MP.

Ms Gallagher threw the challenge down to Mr Abbott on Thursday after the High Court overturned the ACT law.

"This is not a campaign he can avoid," Ms Gallagher said.

"Over to you, Mr Abbott".

Dozens of couples who married in a five-day window before Thursdays ruling have now had their marriages declared invalid.

The Abbott government swooped on the laws the moment they were passed in October, immediately launching action through the High Court to have the Act overturned.

Attorney-General George Brandis urged the ACT government at the time to wait for the outcome of the High Court challenge before allowing couples to marry.

But a defiant ACT government refused and at least 27 couples have married in the ACT since Saturday.

Reaction to the High Court's decision

ACT Attorney-General Simon Corbell said that Prime Minister Tony Abbott should now allow a conscience vote on same-sex marriages in the Federal Parliament.

"Now is the time for a national debate on this issue as we witnessed the joy of those couples that have been married since Saturday, when the first weddings took place under the ACT Governments law," Mr Corbell said.

Western Australian state Labor politician Stephen Dawson, who flew to Canberra to marry his partner Dennis Liddelow in the early hours of Saturday morning, said he had mixed feelings leading up to the High Court's decision.

"I am disappointed in the decision. While my heart said one thing and my head said another leading up to the decision," he said.

"My heart said they've given us this window of opportunity, perhaps it's going to be a positive sign. My head was saying I wasn't sure the territory could legislate in this area," he said.

Democratic Labor Party Senator John Madigan said any decision on the definition of marriage needed to be made by the people, not the courts or the parliament.

"I agree with the High Court's ruling on the ACT legislation - the Federal Marriage act clearly defines marriage as between one man and one woman,'' he said.

"Marriage has always been between a man and a woman and should always remain between one man and one woman."

ACT Greens Member for Molonglo, Shane Rattenbury has called on the Federal Government to now legislate for an end to marriage discrimination after the High Court has ruled the ACT's Marriage Equality Bill unlawful.

"Today's ruling from the High Court was about legal technicalities it was not about the morality, the common sense or the human importance of ending marriage discrimination," said Mr Rattenbury.

Greens Senator Sarah Hanson-Young said the only way to guarantee marriage equality across Australia was to pass reform at a federal level.

''Australia, as a nation, is ready for this and it's time the Federal Parliament recognised that," she said.

Attorney-General George Brandis is expected to give his reaction soon.