A majority of Catholics, Christians and other religious groups support same-sex marriage and are inclined to vote for it in the forthcoming postal survey, according to new polling commissioned by advocates.

The "yes" side starts the campaign with the backing of 66 per cent of all Australians, with support among the non-religious at 79 per cent, compared with 58 per cent among people of faith, the research shows.

Catholics and non-Christian religions were more likely to support same-sex marriage, with two thirds of both those groups indicating they were in favour. For Anglicans, Uniting Church and Church of England, the figure was 59 per cent.

Marriage equality advocates will use the findings to encourage Australia's 5 million Catholics to ignore directives from the church's leadership and instead vote with their conscience.