From the Associated Press:
The federal government had challenged the validity of the Australian Capital Territory's law that had allowed gay marriages in the nation's capital and its surrounding area starting last Saturday.
The High Court unanimously ruled that the ACT's law could not operate concurrently with the federal Marriage Act, which was amended in 2004 to define marriage as between a man and a woman. [SOURCE]
Go read more about the decision and the case leading up to it.
Along with the ruling, the Court explained in a statement that "The Marriage Act does not now provide for the formation or recognition of marriage between same sex couples. The Marriage Act provides that a marriage can be solemnised in Australia only between a man and a woman. That Act is a comprehensive and exhaustive statement of the law of marriage."
This case brings to mind the need here in America to pass a federal Marriage Protection Amendment. If you haven't already contacted your legislators urging them to support the federal Marriage Protection Amendment authored by Representative Tim Huelskamp, please do so today!