A mischievous bedroom billboard mocking the Christian conception story has been defaced just over five hours after it was erected by a church in downtown Auckland.

The controversial billboard, erected by St Matthew-in-the-City Church about 11am today, showed Joseph looking down dejectedly and Mary looking sad. Underneath is a caption, "Poor Joseph. God is a hard act to follow."

The image, which the church said was put up to stir debate about the real meaning of Christmas, was seen defaced with brown paint shortly after 4pm.

"When I drove by at 3.30pm it was fine," St Matthew-in-the-City spokesman Clay Nelson told NZPA.

He said a television cameraman had captured footage of the vandalism and it was likely to be handed to police and a complaint laid.

"We had asked some of the street people to keep an eye on it overnight but it's taken even less time than that," Mr Nelson said.

"We knew this would create a little bit of a fuss but I had no idea that my co-religionists had no sense of humour."

Mr Nelson said the billboard had clearly succeeded in sparking debate, judging by publicity and the number of messages the church had received.

"This is going to be a topic of conversation on dinner tables around the world," he said.

"Getting people to talk about Christmas in real ways is maybe worth a little bit of paint."

He said the church would find a way to get the original image back on the billboard.

Archdeacon Glynn Cardy had said the concept was to get people to think what Christmas was all about.

"Is it about a spiritual male God sending down sperm so a child would be born, or is it about the power of love in our midst as seen in Jesus?"

He said the true importance of Christmas "is in the radical hospitality Jesus offered to the poor, the despised, women, children, and the sick, and says: 'this is the essence of God'. His death was a consequence of the offensive nature of that hospitality and his resurrection a symbolic vindication".

Archdeacon Cardy said the church had asked an advertising agency to come up with a few ideas in November, and that the billboard they chose wasn't the most radical one offered up to them.

"One of the options we turned down had a sperm coming down with the words 'Joy To The World'."

Catholic Church spokeswoman Lyndsay Freer had described the image as inappropriate and disrespectful, while traditional values group Family First said it was wrong to confront children and families with the image.

Last week a campaign by New Zealand Atheist Bus Campaign raised $20,000 in public donations to fund bus ads which read "There's probably no God. Now stop worrying and enjoy your life".

Those ads created a storm when they ran on the London Underground and British buses this year. Similar ads have run in the United States, Canada, Italy, Spain, Australia, Finland and Germany.