The Napier Equippers Church may have to answer to the Advertising Standards Authority if the organisation deems its sign stating "Jesus Heals Cancer" falls within its jurisdiction.
The authority will also determine what codes the sign may breach, before reviewing complaints and allowing the church to respond.
It is investigating the Napier Equippers Church sign after receiving nine complaints.
The Tamatea church erected the billboard last week to the disgust of the local Condin family, whose three-year-old son is undergoing treatment for leukaemia. On Monday the church added a tally to the sign to symbolise the six people it claimed to have seen healed of cancer.
Advertising Standards Authority chief executive Hilary Souter said the complaints asked how appropriate the billboard message was.