The Navy has been working to make four WW2 mines in Whangaparoa harbour safe.
Divers are attaching an explosive charge to each mine and detonating it to break it open.
This will flood the mine and ensure the contents are safe.
The mines were planted as part of a controlled field in WW2. They are sitting about 28 metres deep, a depth which is no longer out of reach of recreational harbour users.
The explosion occurred below the surface, 1 kilometre from Navy Dive Vessel the Manawanui, and did not cause a major splash above surface.
Consideration was given to the wildlife in the popular fishing spot. Special acoustic equipment was used to deter fish and other marine animals from the area.