The ACT parliament passed a bill in October making the territory the first part of Australia to legalise same-sex weddings.
But the national government challenged the decision, saying it was inconsistent with federal laws.
Some 27 couples who married since the law came into effect last weekend will now have their unions declared invalid.
The court said the issue should be decided by parliament – which in September 2012 voted down gay marriage legislation.
see more: http://www.bbc.co.uk/news/world-asia-25344219