Unexploded World War II mines have been detonated by the Royal New Zealand Navy's dive team in the Hauraki Gulf today.
The three underwater mines were discovered during an international mine countermeasures exercise at the beginning of this month.
Navy able diver Kyran Bennett said the exercise was a rewarding part of being a navy diver.
"This is the first time I've ever done anything real time," Bennett said.
"I first dived these mines about two weeks ago. We just basically dive down to identify and make it safe and operate on it."
He said he wasn't sure why the explosion did not send water high into the air as this was the first time he had taken part in something like this.
Operational dive team Warrant Officer Simon Marston said the divers had wanted to dispose of the mines safely without damaging the environment. The amount of explosives was minimised and an anti-mammal device was used to scare off any mammals in the area.
"The mines are spherical in shape and [are] command-detonated mines," Marston said. "They were connected to each other using a command cable which connected them to the shore."
The danger was mitigated through extensive training, he said.
Hundreds of mines were laid defensively to protect harbours during the war.
German raiders planted dozens of mines in the Hauraki Gulf. One mine planted by the raider Orion sank the trans-Pacific liner Niagara off Bream Bay in Northland.
No-one was killed although the record shows the ship's cat, Aussie, did not make it off.
Niagara was carrying a secret cargo of gold that would be worth $230 million today. It was successfully salvaged.
Another Orion mine struck a converted minesweeper, HMNZS Puriri. The ship sank so quickly that five men, including its commanding officer, drowned.
The area where the navy detonated the mines today was well known and is marked on nautical charts as an "explosives dumping ground".