Ministers from Catholic and Protestant churches have joined forces to urge Parliament to vote against a bill on same-sex marriage this evening.
A joint statement was signed by 70 ministers including national heads of church denominations.
Reverend Richard Waugh, from East City Wesleyan church, said the group wanted MPs to be in no doubt about what mainstream Christian churches felt about the bill.
"The State should not presume to re-engineer a basic human institution. The complementary role of male and female is basic to the very character of marriage, along with having and raising children.
"Same-sex relationships are intrinsically different, so can never be regarded as true marriage."
He said the Prime Minister and other MPs had assured the public in 2004 that marriage would remain a sacred union, and that civil unions were an acceptable alternative.
"It is now time for Members of Parliament to recall and honour those assurances."