Victoria's six Catholic bishops will circulate 80,000 letters this weekend asking their parishioners to show the federal government their opposition to same sex marriage.
There are currently three gay marriage private member's bills before federal parliament, aimed at changing the legal definition of marriage as a union between a man and a woman.
The bishops want all Catholics to contact their MPs and respond to an online survey being conducted by the Federal Parliament Standing Committee of Social Policy and Legal Affairs.
The Bishop of Sale, Christopher Prowse, said it would be a grave mistake with implications for the future of society should the legal definition of marriage be changed.
"We have asked Catholics to seriously reflect and pray about the ramifications for current and future generations of legislation which completely redefines marriage," Bishop Prowse said.
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Marriage equality supporters have described the church's campaign as "alarmist" and rejected claims gay marriage would undermine family life or damage society.
"Families and societies are only strengthened when couples are allowed to commit to each other through marriage," national convenor of Australian Marriage Equality Alex Greenwich said.
"So to hear Archbishop Hart discouraging any recognition of this commitment is extraordinary and heartless."
A private bill, amending the Marriage Act to include same-sex couples, has been introduced to federal parliament by Labor MP Stephen Jones. Another bill is being jointly proposed by Australian Greens MP Adam Bandt and independent Andrew Wilkie.
Both bills have been referred to parliamentary committees for detailed examination.
A third bill, proposed by the Greens, will be considered in the Senate. Former NSW premier Kristina Keneally, a devout Catholic, said people of her faith should look at a range of information sources to formulate their views.
"I've come to a position, with a fully-formed conscience, that I support gay marriage," she told ABC Television. "I would encourage all Catholics to apply critical thinking to this issue."
Ms Keneally said the teachings of the church were not infallible although it was important people take heed of what their parish priest or bishop was saying. "But it's equally important for them to consider how they in good conscience must act."
