The survey is non-binding but PM Malcolm Turnbull has vowed to hold a vote in parliament if the majority of Australians choose 'yes'.
Sydney: Australia's prime minister on Friday defended a church's right to not marry some couples following reports of a woman's wedding being cancelled after she posted support for same-sex marriage on Facebook ahead of a national vote.
A voluntary postal ballot involving up to 15 million Australians on whether gay marriage should be legalised is underway, with the results due in mid-November.
The survey is non-binding but Prime Minister Malcolm Turnbull has vowed to hold a vote in parliament if the majority of Australians choose "yes".
The poll has attracted heated debate from the "yes" and "no" campaigns, with some prominent clergy speaking out against such unions, warning that legalising them could infringe on religious freedom.
Fairfax Media reported Friday that one couple were due to be married in the rural town of Ballarat in Victoria, but after the bride-to-be posted support for changing marriage laws on Facebook she was told the church minister would no longer officiate.
"You must surely appreciate that your commitment to same-sex marriage opposes the teaching of Christ Jesus and the scriptural position practised by the Presbyterian Church of Australia and by me," the minister wrote in a letter to the bride provided to Fairfax.
"This conflict of views has practical consequences in relation to your upcoming wedding. By continuing to officiate it would appear... that I support your views on same-sex marriage or that I am uncaring about this matter."
The church, Ebenezer St John's, had no immediate comment Friday, but Turnbull defended the minister's decision, saying "churches are free to marry whoever they like".
"Churches are entitled to marry or not marry whom they please. That is part of religious freedom. My own church, the Catholic Church, will not marry someone who has married before," he told reporters in Canberra.
Turnbull, a moderate who supports same-sex marriage, is opposed by some members of his conservative ruling Liberal-National coalition on the issue and the postal vote is seen as a compromise.
"No" campaigners say changing the laws could impact the church, but both Turnbull and Labor opposition leader Bill Shorten -- who also supports gay marriage -- have said religious freedom would be protected under any reforms.
Parliament on Wednesday passed election-style safeguards restricting campaign material that might be misleading and deceptive during the ballot period, amid fears the vote could unleash a barrage of homophobia.