The Catholic Archbishop of Sydney has urged parents whose children attend Catholic schools to vote against same sex marriage, saying that doing so does not mean they are bigots.

The message from Archbishop Anthony Fisher came in a joint letter written with the executive director of Sydney Catholic Schools, Dan White, encouraging parents to vote 'No' in the national postal survey on same-sex marriage.

'Saying "no" to same-sex marriage doesn't mean that you love anyone any less,' the pair said in the letter signed on September 15.

They added that 'the voices of ordinary Australians being drowned out' in the public debate.

'Sadly, there has been a campaign to label all those who support traditional marriage as "bigots" and "homophobes", so as to silence all opposition,' Archbishop Fisher and Dr White said.

They continued: 'Most of us know someone who is same-sex attracted and we want only the best for them. But we also love marriage as traditionally understood and want to maintain what is special about this relationship.'

They also claimed that faith-based schools have much to lose if same-sex marriage is endorsed.

'It is important to realise that no protections of our freedoms have been proposed so far by those who are pressing us to vote,' the letter said.

'We are being asked to vote blind, without seeing what changes will be made to the law and what rights, if any, will be guaranteed.'