SYDNEY (Reuters) - Thousands of people rallied for marriage equality in Australia s second-biggest city of Melbourne on Saturday ahead of a postal survey on same-sex marriage which could lead to its legalisation. Australia is one of the only developed English-speaking countries not to have legalised same-sex marriage, despite strong popular support and the backing of a majority of lawmakers. Shadow Attorney-General Mark Dreyfus, of the opposition Labor Party, called on the conservative Liberal Party-led government to do more to ensure the debate did not turn ugly ahead of the postal survey next month.  I m particularly calling on the prime minister of Australia to speak out against any bile or hate speech that we might see in this campaign,  he told the rally. Prime Minister Malcolm Turnbull last week urged supporters and opponents of same-sex marriage to show mutual respect as their campaigns turned increasingly vitriolic. [nL4N1LB1TJ] Rally organiser Anthony Wallace from activist group Equal Love said 15,000 people attended the event, making it one of the largest gay rights rallies in Australian history. Police declined to estimate the size of the crowd. The rally is an annual event, which this year began and ended at the Victorian State Library, where a mass same-sex wedding ceremony was held.  Australians will vote over several weeks from mid-September in the non-compulsory postal ballot on whether to legalise same-sex marriage. Same-sex marriage is supported by 61 percent of Australians, a 2016 Gallup opinion poll showed, but the issue has fractured the Turnbull government and damaged his standing with voters, now at a six-month low.  