Public access to the area, at the northern end of the bay near Te Kao, was limited but Conrad urged locals to stay away until the explosive had been defused or blown up.
The bomb squad is due at the scene this evening.
Te Aupōuri’s taiao team had been on their way to check a sperm whale that had washed up on the northern side of Parengarenga Harbour when kaitiaki, or guardian, Rangi Everitt spotted the explosive.
Conrad said it was similar to a device he had found at North Cape some years earlier and appeared to have been in the water for some time.
A NZDF spokesperson said their Explosive Ordnance Disposal team was responding to the situation and that it was too early to comment on the exact nature of the device.
“Anyone who discovers a potentially dangerous item should always exercise caution until the item is deemed safe by an expert.
“If you do discover something that might be dangerous, remain clear of the item and contact New Zealand Police as soon as possible,” the spokesperson said.
Additional reporting RNZ