The MP behind New Zealand's same-sex marriage law has slammed the Catholic Church for getting involved in Australia's postal survey on the issue, saying it is "appalling" that the church is casting judgment on same-sex relationships given its record of child sexual abuse.

Louisa Wall is the Labour member for Manurewa, an electorate in South Auckland. She authored the same-sex marriage bill that was passed into law by the New Zealand parliament in 2013, making it the first country in the Asia Pacific to grant same-sex couples the right to marry.

She told the BuzzFeed political podcast Is It On? that she was perplexed by the prominent role of the Catholic Church and its leaders in Australia's marriage survey.

"I can’t understand why they haven’t been told to not lead the 'no' campaign," Wall said. "They don’t have any moral authority. How can you, when your institution over 70 years actively covered up all the sexual abuse of children?

"The process they’re leading is affecting all the LGBT young people. It’s just disgusting."