Sydney - Australia will hold a voluntary postal vote on whether to legalise gay marriage if parliament rejects plans for a plebiscite on the contentious issue, the prime minister said on Tuesday.Parliament's upper house, the Senate, last November rebuffed plans for a national plebiscite involving 15 million people, with the Labour opposition, Greens and cross-bench MPs arguing it would be expensive and spark divisive debate.Several gay senators made impassioned pleas against the plan at the time, saying it would lead to denigration of their families and subject them to hate speech.Many gay rights campaigners agree and instead favour a free vote among MPs in parliament, with politicians not restricted to party policy.But Malcolm Turnbull's government made an election pledge to hold a plebiscite and he plans to send the legislation back to the Senate as early as this week.The ruling Liberals do not have an majority in the upper house, meaning it faces almost certain defeat again.Postal voteIf this happens, Turnbull has committed Aus$122m to a postal vote, run by the Australian Bureau of Statistics, with ballot papers sent out on September 12 and due back by November 7.He said that if there was a majority "yes" the government would bring to parliament's lower house a private bill, on which Liberals could cast a free vote. If the vote was "no", nothing more would happen."Strong leaders carry out their promises. Weak leaders break them," Turnbull said, rejecting suggestions that a postal ballot would break his promise to hold a plebiscite.Voting in elections is compulsory in Australia and would also be so in any plebiscite."I'm a strong leader. I made that promise again and again ... on the campaign trail. You heard me say again and again that every Australian will have a say on this issue," added Turnbull."Now, we have sought to have a compulsory attendance plebiscite. The only reason it has not been held is because of [Labour leader] Bill Shorten's opposition."Debate on gay marriage has lasted more than a decade in Australia.Despite strong popular support for marriage equality, the country is seen as lagging behind nations that allow homosexual couples the right to wed.Same-sex couples can have civil unions or register their relationships in most states across Australia, but the government does not consider them married under national law.The Australian Marriage Equality organisation made it clear it still wants to see a free vote in parliament."The plebiscite is a policy that failed last year. It is a policy that will fail again," it tweeted. "A conscience vote is the only respectful way forward."