The main Islamic organisation, the Australian Federation of Islamic Councils, has backed an election day poll on gay marriage – but also questioned the right of non-believers to cast a vote on the ''religious institution'' of marriage.

Keysar Trad, the assistant secretary of the Federation of Islamic Councils, said: ''Let's have a referendum but a referendum of this nature is a little strange if people who don't believe in God are allowed to tell people who do believe in God about a religious matter.

''Marriage is a religious union under God, it has always been and should always be. God sets the rules, not Parliament.''

Christian groups, led by the Reverend Fred Nile, are calling for a referendum on gay marriage, saying Australians will reject any change to the status quo.

But Mr Nile insisted a referendum question must be a ''black-and-white choice'' on whether to allow ''homosexuals to get married'' rather than a generic commitment to support ''marriage equality''.

''I'm in favour of marriage equality – between a husband and a wife,'' he said.

''I think people should decide the issue. But the question has to be clear. A question like 'are you in favour of marriage equality?' will confuse some people.

''The question has to be black and white: Do you agree that homosexuals should be legally married?

''I think the majority of people would vote no if the question was clear.''

As same-sex marriage advocates voiced their fears about a divisive referendum, Mr Nile joined the Australian Christian Lobby in calling for the matter to be settled by the public on election day.

He said his Christian Democratic Party had been ''pipped at the post'' by the independent Tony Windsor in calling for a question on gay marriage to be added to a referendum, expected to be announced soon, on constitutional recognition for local government.

Marriage equality activists are concerned that what they describe as ''cashed-up'' church groups would bring on a pre-referendum battle that would polarise society and ''demonise'' gay and lesbian people. Mr Nile and Australian Christian Lobby spokesman Lyle Shelton confirmed significant resources would be thrown behind any ''no'' campaign.

''This is something that people are very passionate about,'' Mr Shelton said.

Before Labor's 2011 national conference, the group collected more than 100,000 signatures against gay marriage and Mr Shelton predicted a referendum would fail.

''I would think that if the arguments were presented in a balanced way without the accusations of homophobia or bigotry that are often put towards those who support marriage, I think there would be every chance that people would stick with what has been the status quo for millennia,'' he said.

The stance of the Christian groups exposed divisions in the gay marriage lobby. Fairfax Media understands that Greens senator Sarah Hanson-Young, who has portfolio responsibility for marriage equality, lobbied leader Christine Milne to reverse her support for a referendum.

In comments that confirmed she had been forced into a U-turn, Senator Milne said a referendum would be a ''distraction'' and the issue should be settled by Parliament. ''The only impediment is that the Coalition won't provide a conscience vote,'' she said.

Australian Marriage Equality national convener Rodney Croome said: ''We fear cashed-up opponents of marriage equality would exploit a referendum to polarise the electorate and demonise gay and lesbian people in a way that will impact badly, particularly on young gay people.''

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