NZDF Navy divers and explosives team neutralise 60-year-old depth charge threat in the waters off Whangaparaoa, Auckland

There was no way of telling if a metre-long bomb, found in the waters off Whangaparaoa Peninsula, north of Auckland, was full of concrete – or enough explosives to sink a submarine.

A 50/50 mix of live depth charges and "drill rounds" were fired liberally around New Zealand after World War II, and they look exactly the same on the outside.

So when an ominous Mark 10 anti-submarine mortar was spotted on the sea floor in Army Bay over the weekend, the New Zealand Defence Force had no choice but to treat it as a threat to human life and dispose of it.

NZDF The damaged casing was all that was left of a potential depth charge after dive crews blew it up.

But the potential bomb could not be physically moved for fear of accidental detonation.

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This depth charge in particular was most likely programmed to explode at about 10m deep and it was sitting at 8m, Lieutenant Wesley Moir, head of one of the mine counter measures teams, said.

NZDF The bomb was found one nautical mile off the Whangaparaoa Peninsula, north of Auckland.

So Operation Poseidon, a plan to blow it up underwater, was hatched.

NZDF explosive teams and Navy divers met at the Army Bay boat ramp on Thursday where they assembled a "small quantity" of explosives, Moir said.

"If it is live ... we should see a small fountain of water and hear a large bang."

NZDF A diver located the potential depth charge and attached a small explosive on top of it.

The ideal situation was the bomb would turn out to be a dud, Moir said.

That would minimise the impact on the surrounding environment and sea life.

First, police's Deodar launch and the Harbourmaster formed a cordon about 1.3km back from the depth charge to stop unsuspecting civilian boats passing through.

NZDF Just the nose and the tail remained of the potential depth charge - the body was blown to bits.

The Navy then set off a series of bangs loud enough to scare away any passing marine mammals, such as whales and dolphins.

One of the divers then plunged into the cold water and found the explosive about one nautical mile off the coast.

It was jammed in a rock ledge in covered in barnacles and crustaceans and had potentially been there for 60 years or more.

NZDF The mission was deemed a success, and the bomb threat neutralised.

The diver then placed 2kg of plastic explosives on top of it, with a long fuse, and got well back.

The explosion caused only a small splash, indicating the bomb was a drill round, and the threat was declared officially neutralised.

Moir said depth charges were primarily used in New Zealand from 1955 to 1980 during training in case of covert submarine attack.

Live bombs could contain up to 90kg of explosives and put a hole in the skin of a submarine.

The drill rounds, however, were filled with concrete and only a small quantity of explosives, so crews could spot where they landed.

About 150 military explosives were found on land every year, but depth charges were a lot less common, Moir said.

This was the first one found in several years.

"It was a good day on the water and a successful mission. Damage to the environment was minimal, if any," he said.