The federal government has again vowed to block the ACT's civil union legislation if it includes a clause allowing same-sex couples to have a public ceremony.

Attorney-general Robert McClelland told The Australian newspaper today that the Rudd government considered the "ceremonial aspects of the ACT model" to be inappropriate.

"We think a civil unions register along the lines of Tasmania is appropriate," he said.

But the ACT attorney-general Simon Corbell is equally adamant that the territory will not change its mind. He told a gay rights rally in Canberra last weekend that the government would stand its ground.

The stand-off has prompted Greens leader Senator Bob Brown to come out in support of the ACT's legislation.

"The Rudd government should not interfere," the senator said.

"Unlike Chief minister Jon Stanhope who campaigned and was elected on the issue of civil unions, the Rudd government has no mandate to interfere with them."

"I wholeheartedly support the ACT civil union legislation. The Rudd government should remove all discrimination on the basis of sexuality under the law, including marriage."

The Greens leader introduced a bill in 2006, called the Australian Capital Territory (Self-Government) Amendment (Disallowance Power of the Commonwealth) Bill, which would allow the Senate to remove the power of the executive to override ACT laws.

It is still being considered by the parliament.