The most senior minster to speak out on the issue was Senator Abetz' - who said Australia should not legalise gay marriage because no Asian country had done so and warned the proposed change could lead to polyamory - but publicly and privately, conservative MPs are mobilising to block change. Christine Forster, sister of Tony Abbott, wants Liberal MPs to have a free vote. Credit:Steven Siewert At least six members of the executive, including cabinet ministers Malcolm Turnbull and Greg Hunt and junior ministers Simon Birmingham, Josh Frydenberg, Marise Payne and Kelly O'Dwyer, support the landmark social reform and would have to quit if the call from Senator Abetz was heeded. Mr Abbott pushed back against those calling for debate, and a free vote, in the Liberal party room on Thursday declaring it was "quite unusual for private member's bills to come on for debate". "This government was elected to grow jobs, to promote the stronger economy, to secure our nation against the various challenges that we face at home and abroad, and that's what we are doing," he said.

Mr Abbott's comments were interpreted by many Liberals as a clear sign that the House Selection Committee, which chooses what bills come forward, would block debate. Prime Minister Tony Abbott on Thursday. Credit:James Brickwood But Ms Forster, a Liberal City of Sydney Councillor who is gay and a strong supporter of the reform, said her view was Liberals should be granted a conscience vote, and that clearly "debate is now raging between opponents and proponents". "The cross-party private member's bill needs to be listed for debate in the parliament, which would in turn trigger discussion within the party room. If that doesn't happen it's my view the issue needs to be raised for debate in the party room," she said. Government leader in the Senate Eric Abetz opposes same-sex marriage. Credit:Andrew Meares

"This is something that as a party we need to define – and hopefully redefine it – our position on, that marriage is an issue on which members can exercise their consciences." Mr Abbott's comments angered some Liberal MPs, who pointed out that he had pledged to have a party room debate on the matter in this term of government. Illustration: Ron Tandberg "If he learned anything from February, the party room is not a play thing for the Prime Minister," one Liberal MP said. "I think he would look like a liar if he didn't [allow a debate to go ahead]" and that the Liberal Party would look "ridiculous" if it did not back a free vote.

One government frontbencher said Coalition MPs would absolutely be granted a free vote on same-sex marriage and laughed off Senator Abetz's call to quit. But another minister, who also asked not to be named, delivered a stinging rebuke to proponents, warning "this is Liberal Party policy and the law of the land, why should I help people who want change and don't have the balls to change the law, and who want to do it behind closed doors?" A conservative power broker said MPs' pre-selection was a "sleeper issue" and that in Queensland, NSW, Tasmania and WA, MPs could find their positions under threat from the party's conservative membership. Fairfax Media estimates that if a free vote were allowed on the bill in the Lower House, at least 65 MPs from all sides of politics would vote for it, with 76 needed to pass it. Australian Marriage Equality says that adding in undeclared supporters, there are an estimated 73 votes in the lower house and a slim majority of two votes in the Senate.

National director Rodney Croome said "the Prime Minister said he wanted a bill that is 'owned by the whole Parliament' [in May] and that is what this bill is." with Latika Bourke Follow us on Twitter Follow James Massola on Facebook