Australia's Prime Minister Malcolm Turnbull has defended churches' rights to refuse to marry people as the country's debate on same-sex marriage heats up.

Turnbull, a strong supporter of legalising gay marriage, vowed to support religious freedom and churches' ability to hold wedding ceremonies for who they want.

'Churches are free to marry whoever they like,' Turnbull said on Friday according to a press conference transcript in First Post.

'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.'

It comes after a church in the southern state of Victoria told a young couple they could not get married in their building after the bride posted a message on Facebook support gay marriage.

The comments come as Australia's debate around same-sex marriage becomes increasingly fractious and threatens to divide the delicately balanced Liberal-National coalition government as well as the 24 million population along religious and generational lines.

A gay Queensland university student speaking in favour of the 'no' campaign was heckled and abused in a video shared by former Prime Minister Tony Abbott, who strongly opposes gay marriage.

'We're here today because we support marriage as it has always been, between one man and one woman,' the man says at the demonstration at the University of Queensland.

'I am here, specifically, because I'm gay and I am standing up against them.

'They want to drown us out. They want to drown me out. They want to speak for me. They want to speak for me because I'm gay and I am standing up against them.'

He goes on to claim there are 'thousands' of gay Australians like him who are against same-sex marriage because of its effects 'on the family, on schools, on politics, on churches' but are vilified for their views.

'These people hate us. They call us Nazis, bigots, homophobes. Where is the real hatred?' he says.

The demonstration last Monday comes as thousands reacted to a 'vote no' message written across the sky over Sydney on Sunday.

The anonymous author of the GoFundMe page said it was 'time for traditional Australian's (sic) to take a stand'.

'It's time we all sent a clear message that we will not put up with our way of life been (sic) deconstructed any further,' the page said.

But the site was inundated with messages of fury.

'I feel sorry for all of you,' one woman wrote.

'What an awful way to live your lives. I can't imagine being so hateful.'