The right mix of weather conditions carried the effects of 500lb (227kg) bomb explosions from north of Auckland on Wednesday, shaking homes and rattling eardrums in Devonport, 70km away.
The air force dropped 12 bombs - each one equivalent to 440 sticks of dynamite - during training in their Kaipara Bomb Range, mostly around lunchtime and in the afternoon.
They were felt and heard in the Waitakere Ranges, Stanmore Bay, and Albany. Normally the sound has a radius of 10 to 15km.
Air Vice-Marshal Mike Yardley, Chief of the Air Force, said it was routine training that had been publicly notified, but he was concerned that it had caused such a stir so far afield.
"We're not intending to frighten the public, that's for sure, and even if we put out notices in the Herald, not everyone reads the classifieds and this would have come as a surprise."
The 500lb bomb has an explosive component of 150lb.
"It's an unusual noise, and a very large noise.
"This is the first time in my 30 years in the Air Force that I've known there to be an interest greater than in the immediate vicinity.
"It is very unusual - an amazing weather phenomenon."
MetService meteorologist John Law said a decent westerly wind, humidity and low clouds all conspired to carry the noise from the coast to the city.
"At Whenuapai, the southern end of the range, there was a westerly of about 30 km/h, and low cloud, which would have helped reflect the sound between the ground and the cloud, propagating it further to the east.
"It acted like a channel for the sound, as it were."
He said humidity - about 70 per cent - and density in the atmosphere also helped to push the sound.
"It's not unusual. It's just that if the conditions are right, you can hear things a lot further away."
Air Vice-Marshal Yardley said the air force always took the weather into account, but not normally with regards to the sound.
"There are optimal weather conditions for training, but it's more to do with bad weather at the range and our ability to ensure safety.
"Having a cloud layer and a light westerly wind is certainly not something that would normally deter us from dropping bombs.
"But it's certainly something we will take into account now."
The exercise is to hit targets in the sand dunes from a P-3K Orion, at 1000 feet.
The Orion flies at around 330km/h when the bombs are dropped.
"We were very pleased with the results [on Wednesday] - about 50m from the mark, which is very accurate for a non-guided bomb."
He said the training has taken place annually for the last 15 to 20 years.
It happens over a one to two-week period, and the bombing area is 300sq m of controlled sand dunes.
The training runs had a perfect safety record, he said.
Air Vice-Marshal Yardley said the last time the bombs were used in a field operation would have been in 1956 in Malaysia, when New Zealand helped the British fight Malayan attempts to overthrow the colonial administration. "New Zealand is not well known for going out and bombing countries, or being invaded."
The training is expected to continue on Monday and Tuesday, and to be completed on Wednesday.
Mr Law said a high pressure system should move in early next week, which would not carry the sound with the same force or direction as yesterday.
- APNZ
