Opposition MPs in Australia say they will block a government decision to hold a national vote on legalising same-sex marriage early next year.

Critics, including supporters of same-sex marriage, have instead said the issue should be decided in Parliament and not by prime minister Malcolm Turnbull's plan of a national vote.

Mr Turnbull's conservative coalition government requires the support of the centre-left Labour Party before it can hold a the non-binding vote on gay marriage on February 11.

However, opposition leader Bill Shorten said a meeting of Labour leaders had unanimously decided to block it - arguing that a plebiscite would lead to homophobic rhetoric and a nasty public debate.

"This country does not have the right in a plebiscite to pass judgment on the marriages and relationships of some of our fellow Australians. It is not what Australia is about," Mr Shorten said.

Mr Turnbull, who supports marriage equality, has said he will continue to push for the national vote and has urged senators to "give the people their say."

Meanwhile, anti-marriage equality campaign group the Australian Christian Lobby said it was disappointed "that ordinary Australians are being shut out from having a say about the biggest social policy change in a generation".

Most Australians support marriage equality, according to opinion polls.