Most Australian Christians, including Catholics – alongside other religious groups – are expected to back gay marriage in the nation's upcoming referendum, according to a new survey.

Among those of faith, 58 per cent support the 'yes' campaign, supporting the legalisation of gay marriage in Australia – where it currently is not legal.

Catholics were the most supportive for the proposal, with as many as two-thirds saying they would vote 'yes' – matching the proportion of non-Christian religious voters who said the same.

Among the Anglican and Uniting Church denominations, 59 per cent backed a 'yes' vote. Among all Australians, 66 per cent of the 1,000 polled supported gay marriage, while non-religious voters showed the most support at 79 per cent.

The survey was completed by the Jim Reed of Newgate Research, according to The Sydney Morning Herald.

Australia's non-binding postal-plebiscite will seek the will of the people on the question of gay marriage beginning September 12 and concluding November 7.

'The upper management of the church isn't listening to the flock,' said Tiernan Brady, the executive director of the Equality Campaign, who commissioned the poll.

'The flock have made up their mind on this, they support marriage equality.'

Last week saw several Christian leaders speaking out ahead of the national vote, with some Catholics warning of a threat to religious freedom.

Hillsong's Brian Houston warned of 'wide-ranging ramifications' for changing the definition of marriage, but urged all Australians to vote whatever their conscience might say on the issue.

The Catholic Archbishop of Melbourne, Denis Hart, said that its 180,000 employees such as teachers and nurses would be fired if they had same-sex marriages.

'I would be very emphatic that our schools, our parishes exist to teach a Catholic view of marriage,' Hart told Fairfax Media. 'Any words or actions which work contrary to that would be viewed very seriously.

'Our teachers, our parish employees are expected totally to uphold the Catholic faith and what we believe about marriage. People have to see in words and in example that our teaching of marriage is underlined.'