In a letter sent to Mr O'Farrell on Monday night, Mr Nile said he was ''very upset at the Premier's public support for homosexual marriage and his challenge to Tony Abbott's rejection of a conscience vote on the Homosexual Marriage Bill.

"Our Christian Democratic Party will withdraw its support for O'Farrell legislation until his challenges to Tony Abbott's policy cease.''

The Premier last week said he had changed his mind on the issue and now believes same-sex marriage should be given legal status under the federal Marriage Act.

Mr O'Farrell also said the Opposition Leader, Tony Abbott, should allow a conscience vote on the issue.

"The Premier needs to show some loyalty to Tony Abbott," Mr Nile said.

Mr O'Farrell came out publicly in favour of same-sex marriage after the New Zealand Parliament voted to change its laws. ''My view - a view that I've come to in recent years - is that as a Liberal who believes that commitment and family units are one of the best ways in which society is organised, I support the concept of same-sex marriage,'' Mr O'Farrell said.

Asked about Mr Abbott's position, Mr O'Farrell argued Federal Parliament ''ought to reflect the community and allow same-sex marriage. I think in coming to that decision the Federal Parliament should do so by way of a conscience vote across all parties.''

The O'Farrell government has been dependent on Mr Nile's vote to pass legislation to privatise state electricity assets.

A NSW upper house inquiry is considering whether the state is constitutionally able to legalise same-sex marriage.

In November, a cross-party working group gave notice it would introduce the State Marriage Equality Bill to the upper house.

Last May the NSW upper house voted 22-16 in favour of a motion supporting same-sex equality, but many believe any attempt to legalise same-sex marriage would be defeated in the lower house.