The Presbyterian Church of NSW has discussed withdrawing from the Marriage Act if it is amended to include couples of the same sex, saying it would be better to "avoid association with evil by no longer acting as celebrants", a document obtained by SBS has revealed.

At last week's assembly - the annual meeting of all Presbyterian churches in the NSW - the church voted to ask its federal body to take the necessary steps to withdraw from the Act should the law change.

The motion was passed by a vote of 140 to 62.

Other Christian denominations are also investigating their options should the Act change, with some ministers understood to be backing the Presbyterian move.

Should the Presbyterian Church federal body agree, it would mean its ministers would no longer be able to solemnise marriages. The church argues the marriage would be recognised by God, but not by the government unless a separate civil ceremony was held.

It could also lead to two types of marriage - a church marriage, which is not recognised by the state, and the state-sanctioned marriage.

A statement from the church's NSW Assembly moderator, the Reverend Kevin Murray, was read or distributed to all Presbyterian Church members in NSW yesterday.

It upheld the church's position that marriage is a "life long covenant between one man and one woman".

"Jesus clearly teaches that marriage is between a man and a woman," it reads.

"The Assembly considered what the church should do if marriage is redefined in Australia.

"It decided to ask the General Assembly of Australia to withdraw the whole church from the Marriage Act, so that our ministers could no longer solemnise marriages under the Marriage Act.

"The report which recommended this decision argued that if the Federal Government were to redefine marriage to include same-sex marriage then it would corrupt a good gift of God into a wrong.

"That would mean that ministers would then be acting for the government in a system which did not reflect the biblical view of marriage.

"In this case the positive reason for our co-operation with the Marriage Act would have been removed, and we would be better to avoid association with evil by no longer acting as celebrants."

Rev. Murray stressed the decision was not yet final, but the General Assembly of Australia was still to consider the issue.

The federal body is not due to meet until September next year, but an emergency meeting could be called.

The statement said other state assemblies were having similar discussions and could also ask the General Assembly to act.

John McCLean, of the NSW Presbyterian Committee on Society and Culture, today told SBS the issue came down to the Bible's interpretation of marriage.

"I hope it's not a dummy spit," he said. "I hope it's an act of conviction and it was certainly a long and serious debate. It wasn’t as if we just decided on the spur of the moment, it's been something we've been talking about and will continue to talk about for quite a long time to come."