The Australian government has said it will oppose any new laws legalising gay civil unions.
Mr Howard's government formalised marriage laws in 2004
Prime Minister John Howard said he did not intend to allow the institution of marriage "to be in any way undermined".
The authorities in the Australian Capital Territory (ACT), which includes the capital Canberra, introduced a bill on same-sex civil unions last week.
If the legislation is passed, it will be the first such law in Australia, but federal government can overturn it.
Australian law formally defines marriage as a union between a man and a woman.
"There is a special place in Australian society for the institution of marriage, as historically understood," said Mr Howard.
Attorney General Philip Ruddock added that if the ACT authorities "seek to portray civil unions as a marriage, in our view, that is quite inappropriate".
"It suggests to people who might be interested in civil union that what they have is a marriage, when in fact it is not," Mr Ruddock said.
The head of the ACT government, John Stanhope, said Mr Ruddock's reaction revealed homophobia in the Howard administration.
"One has to pose the question of whether or not the real reason (for Mr Ruddock's stance) is that there is no place in John Howard's Australia for homosexuals," he told ABC radio.