Four plumes of smoke at South Head in Kaipara Harbour signalled the Royal New Zealand Air Force's (RNZAF) six-monthly live bomb drop this afternoon.

As part of a training exercise for the Air Force to qualify air and ground crews, an RNZAF Orion P-3K2 airplane dropped five 500lbs (230kg) "Mark 82" bombs on to sand dunes at the Kaipara Air Weapons Range.

One of these bombs failed to detonate, and forced the Orion P-3K2 to take a different route for the final two bombs because of the danger of a delayed explosion.

But Simon Eichelbaum, RNZAF's Public Affairs Officer, explained that it wasn't a failure.


"All of this is part of the training," he said.

The Mark 82 is a non-guided bomb used for anti-submarine warfare. "Essentially gravity seizes the weapon, brings it to the water and then it explodes," said James Arnott-Steel, an RNZAF flight lieutenant.

Unlike in the field, the bombs were detonated on the dunes rather than in the water.

"Dropping it on the land allows us to see it explode. It gives us a lot of data. It also helps if it doesn't explode because then we can go out and get rid of the weapon," said Mr Arnott-Steel.

For those watching there was a slight pause between when the bombs landed and when the bombs detonated. This was expected however, because Mark 82s sink into the water before exploding to cause enemy submarines the maximum possible damage.

Despite the five bombs being dropped in less than an hour, the preparation behind the exercise was very intense.

"Bomb training exercises involve a very long string of events," said Mr Eichelbaum.

"It starts with transporting the weapons, then preparing the weapons, loading them on to the aircraft. When it's flying there's a huge number of checklists to go through."

Along with the transportation of the bombs, Mr Arnott-Steel said the area of dunes has to be inspected.

"Before we do our bombing we have patrols out there to make sure the range is clear," he said.

"It's a lot more than just throwing it out the back of the aircraft," said Mr Eichelbaum. "It's different when it's a real weapon."

Although RNZAF has never used the Mark 82 against enemy submarines, Mr Arnott-Steel said, behind the training the Air Force's "primary aim is always to defend New Zealand".

"There are also upcoming exercises in Hawaii called Rimpac," he said, which is the world's largest international maritime warfare exercise.

The last bomb dropped this afternoon was just 25 metres away from its target.

"If I was a submarine I sure wouldn't be happy," said Mr Eichelbaum.