A church billboard showing an apparently naked Virgin Mary and Joseph in bed together has sparked the ire of conservative Christians in New Zealand.

On the poster a sad-looking Joseph lies next to Mary, whose face is turned heavenwards under the words: "Poor Joseph. God was a hard act to follow."

The billboard was erected outside the progressive St Matthew-in-the-City Anglican church in Auckland yesterday.

St Matthews' vicar, Archdeacon Glynn Cardy, said the billboard was meant to challenge stereotypes about the way Jesus was conceived.

In the bible, the Virgin Mary becomes pregnant after an angel appears to her and tells her she will give birth to the son of God.

Reverend Cardy said the billboard was meant to challenge literal interpretations of the Bible. "It is intended to challenge stereotypes about the way that Jesus was conceived and get people talking about the Christmas story," he said.

Conservative Christians have criticised the billboard as offensive.

Auckland Catholic Diocese spokeswoman Lyndsay Freer said the poster was disrespectful to the church.

"Our Christian tradition of 2,000 years is that Mary remains a virgin and that Jesus is the son of God, not Joseph," Freer told the New Zealand Herald.

"Such a poster is inappropriate and disrespectful."

One protester was so incensed, just hours after the unveiling of the poster, he climbed on top of his car and covered the images of Joseph and Mary with brown paint.