An historic bomb has been detonated by the defence force in Paraparaumu.
Children found the 9cm High Explosive Anti-Aircraft shell during the school holidays three weeks ago in the Belmont Regional Park.
The New Zealand Defence Force said it was full of high explosive TNT.
A police spokesperson says the children had been handling the shell and planned to take it to their school's show-and-tell.
"The kids took it home to their grandparents' house, and after googling the item they thought it was safe to keep and clean it up to take to school for show and tell," Constable Rob Eastham said.
"Last night the children returned to their home in Awatea Ave in Paraparaumu with the artillery round in the car."
An adult saw the children playing with the undetonated round and, concerned it was unsafe, moved it to the backyard. This morning they called the police about it.
Police visited the children's house, evacuated three nearby properties and put in place a safe cordon as a precaution.
The New Zealand Defence Force Bomb Disposal Unit inspected the projectile and confirmed it was potentially still live and should be detonated in a controlled explosion.
The unit carefully transferred the projectile to a safe location at Higgins Quarry in Paraparaumu.
The item was buried under about a metre of gravel in a safe part of the quarry and detonated by remote from 300m away.
A quarry employee says the bang was disappointingly quiet.
The defence force said the Belmont Hills area was used by the army for the firing and disposal of similar armament in the 1940s. The area was decommissioned from this use in 1972.
Eastham said it was a reminder that anyone who finds such an item should contact police straight away and avoid touching it.