SYDNEY (REUTERS) - Australia's Prime Minister Malcolm Turnbull, a strong supporter of gay marriage, has defended the right of a church to refuse to marry a young couple who had posted support for same-sex unions on social media.

The minister of a Presbyterian church in the southern state of Victoria told a young couple in their 20s that they would not be allowed to hold their ceremony at the church after the bride posted a message on Facebook supporting same-sex marriage.

Australia is in the midst of a non-compulsory, non-binding poll to inform Parliament on whether it should become the 25th country to legalise same-sex marriage.

The issue has threatened to fracture the ruling Liberal-National coalition government.

"Churches are free to marry whoever they like," Turnbull said on Friday (Sept 15) in Canberra according to a press conference transcript. "As strongly as I believe in the right of same-sex couples to marry... Religious freedom is fundamental and it will be protected in any Bill that emerges from this Parliament."

Religious freedom in Australia is guaranteed under the Constitution.

The issue has dogged Prime Minister Turnbull for two years as progressive and conservative members of his ruling coalition have pulled in opposite directions, threatening his narrow Parliamentary majority.

Australians have started receiving postal ballots for the poll, which runs until the end of October, and the issue is dividing the country's population of 24 million people along religious and generational lines.

Despite securing 70 per cent public support in an Ipsos/Fairfax poll, the issue of same-sex marriage had faced a political deadlock, broken only last week when the High Court gave the all-clear for the vote.

Fissures have opened within religions as some churches urge their followers to vote against same-sex marriage, while other churches have taken a neutral stance.

The Catholic Archbishop of Sydney, Australia's largest city, wrote to Sydney Catholic school principals on Friday afternoon, quoting Pope Francis and urging them to oppose the vote.

The Bishop of Parramatta, Vincent Long Van Nguyen, however, told his parishioners in a pastoral letter that the vote was a matter of individual conscience.

The Anglican church, which officially opposes gay marriage, has been divided on the issue for years.