SYDNEY (Reuters) - The honeymoon was short-lived for Australia's gay couples who married in the past five days after the High Court overturned new same-sex marriage laws on Thursday, invalidating wedding ceremonies performed since Saturday.

"This is devastating for those couples who married this week and for their families," Australian Marriage Equality National Director Rodney Croome said in a statement.

Around 20 gay couples had tied the knot since December 7, when Australia's first same-sex marriage laws came into force in the Australian Capital Territory (ACT). The first ceremonies were celebrated a minute after midnight (8 a.m. ET Friday).

"I don't want to be unmarried this afternoon," Ivan Hinton, who married his partner Chris Teoh in the national capital Canberra, told Australian media outside the High Court.

Australia's conservative national government had challenged the law in the High Court on the grounds it conflicted with federal law. On Thursday, the court upheld the challenge and unanimously decided that the ACT law was invalid.

The court said the issue of same-sex marriage was a matter for the national parliament. Laws to legalize same-sex marriage failed to pass in the national parliament in September 2012.

"This is just a temporary defeat," said a defiant Croome. "What is far more important is that the ACT's law facilitated the first same-sex marriage on Australian soil and showed the nation the love and commitment of same-sex couples," he said.

Other countries where same-sex marriages are legal include Canada, Spain, Sweden and New Zealand, which in April became the first country in the Asia-Pacific region to legalize same-sex marriage.

(Reporting by Thuy Ong; Editing by Michael Perry)