Owners of homes and businesses built over the water may be exempt from paying rates because of the new Marine and Coastal Area Act, the law drafted to replace the controversial Foreshore and Seabed Act.
The oversight is a boon to some property owners but could cost councils around the country thousands of dollars in lost rates.
Of Northland's territorial authorities, the Far North District Council is likely to be the worst affected.
Properties potentially now exempt include those built out over the water at Mangonui, Paihia, Whangaroa, Rawene and Kohukohu.
Properties at Mangonui include the famous fish-and-chip shop and the home of Mayor Wayne Brown - who is already in a dispute with the council over rates.