SYDNEY (REUTERS) - Australian Prime Minister Tony Abbott's conservative coalition government on Tuesday blocked its members from voting in favour of gay marriage, a politically risky move that effectively rules out a marriage equality bill passing under his government.

The decision by Abbott to use parliamentary tactics to thwart the libertarian wing of his Liberal Party comes just six months after he narrowly survived a party room coup and amid dismal polls that have reignited speculation over his future.

In the Australian parliament, crossing the floor is extremely rare and lawmakers can face severe retribution up to expulsion if they defy the party to vote against their colleagues. The coalition's current position is against same-sex marriage.

Public opinion is strongly in favour of legalising the practice but Abbott, a socially conservative Catholic, has manoeuvred to head off a free vote.

On Tuesday, Abbott called a rare meeting of the full coalition party room to decide the issue. The presence of right-wing coalition partner The Nationals overwhelmed support for a free vote among Abbott's Liberals by a ratio two to one.

"If you support the existing definition of marriage between a man and a woman, the coalition is absolutely on your side but if you would like to see change at some time in the future, the coalition is prepared to make that potentially possible," Abbott told reporters after the near six-hour meeting, suggesting a public referendum on the matter.

"The disposition is that it should happen through a people's vote rather than simply through a parliament vote."

The tactical success over gay marriage could prove politically costly as Abbott struggles to keep his footing following a series of perceived gaffes and amid a sagging economy.

Ireland backed same-sex marriage by a landslide earlier this year in a referendum that marked a dramatic social shift in a traditionally Catholic country that only decriminalised homosexuality two decades ago.

Australia's opposition Labor Party seized on the Irish referendum and days later introduced a bill to legalise same sex marriage, the first by a major Australian political party.

A telephone survey of 1,000 people conducted in Australia a year ago by polling agency Crosby-Textor found support for legalising same-sex marriage at 72 per cent.