Labour MP Louisa Wall has slammed the Australian Catholic Church for endorsing the 'No' campaign during Australia's nationwide postal vote on same-sex marriage.

She says the Catholic Church "don't have any moral authority", because its institution has been involved in covering up the sexual abuse of children for decades.

"The process they're leading is affecting all the LGBT young people. It's just disgusting," she told BuzzFeed's Is It On? podcast.

Ms Wall proposed New Zealand's same-sex marriage bill, passed by a conscience vote in Parliament in 2013.

Australia's Malcolm Turnbull-led government has initiated a nationwide non-binding postal vote on whether same-sex marriage should be legalised, which is currently underway.

Ms Wall told BuzzFeed Australians should stand up to the Catholic Church.

"I wish you'd talk about it and say to them, 'If you want to be a moral crusader, why don't you eliminate child sexual abuse? And be a leader in that'."

In 2012, the New Zealand Catholic Church said marriage should be between a man and a woman.

New Zealand's Catholic Bishops Conference has been approached for comment.

Newshub.