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.