Australia's conservative government has refused to allow its federal politicians to have a free conscience vote on whether to allow gay marriage.

Prime Minister Tony Abbott instead forced his ruling coalition colleagues to follow his party line that marriage should be allowed only between a man and a woman.

Abbott said that Australians would get a chance to vote in a plebiscite on the issue if they re-elected his government next year in a move described by his opponents as a stalling tactic that has all but doomed legislation introduced by the opposition Labor party to allow same-sex unions.

Federal plebiscites, unlike referendums, carry no legal weight.

"The only way to successfully and satisfactorily settle this matter, given that it is so personal and given that so many people have strong feelings on either side of this - the only way to settle it with the least rancour, if you like, is to ask the people to make a choice," Abbott said at a press conference on Wednesday.

"That means that going into the next election, you'll have the Labor Party which wants it to go to a parliamentary vote and you've got the coalition that wants it to go to a people's vote," he said.

After Ireland voted in favour of same-sex unions in May, Abbott had said any decision would be made by the parliament.

The Australian Marriage Equality organisation immediately called for the plebiscite to be held at the next federal election "to give the next government a mandate to enact marriage equality".

"Tony Abbott can gag his party room, but he can’t gag the Australian people who will vote strongly in favour of marriage equality at a plebiscite,” the organisation's national director Rodney Croome said.

A poll last year found that those in favour of equal rights had reached a record high of 72 percent.

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Unlike in countries such as the UK, Ireland and the US, homosexual Australians are only permitted to have civil same-sex unions or register their relationships. The Australian state of Tasmania only recognised the unions as recently as 2010.

Abbott, who once trained to be a Catholic priest, is viewed as one of the most conservative prime ministers Australia has had.

Opposition frontbencher Jason Clare told the AFP news agency that if the Australian people wanted marriage equality, "they are going to have to divorce Tony Abbott at the next election".