RNZAF's bombing run: Behind the mysterious bangs

It began with the roar of an engine, followed by a flash of fire and a plume of smoke, and just when you thought it was all over, it ended with a deafening crack.

The ear-splitting sound of the 500lb Mark 82 high explosive bomb going off hit like a brick wall, as the shock wave sent ripples through our bodies and caused the wooden tower platform we were standing on to tremble.

The gathered media contingent was braving the pouring rain at a military bomb range on the Kaipara Harbour, north-west of Auckland, to witness first-hand what sparked the furore last week after a series of mysterious booms were heard across the region, from Piha in the west to Whangaparaoa in the north-east.

The bangs had city-dwellers confused, with a flurry of calls and emails to newsrooms across the city.

It later emerged the Royal New Zealand Air Force (RNZAF), conducting explosives training, was the source of the thunderclaps.

That the sound of the bombs going off travelled up to 70 kilometres surprised even the air force, which was today on mission damage control, to assure nervous residents the exercise is routine and is carried out safely.

A combination of the wind direction and the low cloud base last Wednesday, coupled with a possible temperature inversion - where the temperature increased at height, instead of cooling, creating a warmer layer - were believed to be responsible for the sound travelling so far, Warrant Officer Garrick Anderson said.

"We were certainly surprised the noise went as far as it did the other day, but rest assured it's certainly done safely. [With] this sort of operation we have safety in mind and we would never continue if that was in doubt."

It has made the Number 5 Squadron - RNZAF's only P3K Orion squadron - reassess how it carries out its annual weapons training. In future, temperature inversion will be taken into account when deciding to go ahead with the bombing, included in the other weather checks.

The RNZAF is also looking at other methods it can use to notify people ahead of such a training exercise, as a notice in newspaper classifieds clearly failed to work ahead of last week's explosives drops.

Six bombs dropped this afternoon - each containing 190lb (87kg) of explosive, with the rest made up of steel casing - were aimed at a flare remotely ignited on the beach, at a point known as HE North (high explosives detonation north).

As the P3K Orion came back around for another drop, we prepared for another ear-drum bashing. But no amount of flinching after the initial explosion can prepare you for just how loud, and how strong, the noise is when it eventually hits.