Kawakawa's famous Hundertwasser-designed public toilets have been closed to the public after the discovery of a cavity in the footpath in front of the building.
A small hole appeared on the edge of the pavement earlier this week, and the council has discovered a metre-long cavity beneath it.
The Far North District Council said it was too soon to say if the hole was related to the settling of the building or a leak from a water main.
It has asked an engineer to investigate, and said the toilets might need to stay closed for a week while the footpath was dug up.
The council said the Hundertwasser toilets had been inspected regularly since 2012 when minor cracks were found in the walls and floor.
It has budgeted $50,000 for repairs to the building this year, and said it would review that if more extensive repairs were needed.
Austrian-born artist Friedensreich Hundertwasser, who designed the toilets, lived near the town for many years before his death in 2000.