The Archbishop of Melbourne has urged Catholics to vote against same-sex marriage "for the health and future" of society.
Denis Hart has written an open letter urging Catholics to vote in the optional postal survey when forms go out in September.
"We sincerely believe that there is a core and fundamental wisdom and truth in the traditional definition and understanding of marriage that should not be ignored and is worth keeping for the health and future of our society," he writes.
He warned future gay marriage legislation could infringe rights of freedom of religion and conscience.
"It could result in restrictions on the right of ministers of religion and religious bodies and organisations having the freedom to teach, preach and speak about marriage between persons of the same sex being contrary to their religious or conscientious beliefs."
Archbishop Hart called for an "active and respectful" debate, urging Catholics to welcome gay Australians as brothers and sisters.
"Like all human beings they are created in the image and likeness of God," he writes.
"They have a right to expect to be loved and welcomed and not subject to unjust discrimination."
He told ABC radio on Wednesday homophobic material disseminated as part of the campaign against same-sex marriage was "totally inappropriate", insisting both sides should express their ideas with conviction instead of creating a campaign of hate.