An historic bomb has been detonated by police in Paraparaumu.

Children found the World War II-era artillery round during the school holidays three weeks ago in the Belmont Regional Park.

A police spokesperson says the children then brought it to their grandparents' house, and after googling the item they thought it was safe to keep it.

"They thought it was safe to keep and clean it up to take to school for show and tell," Constable Rob Eastham said.

An adult saw the children playing with the undetonated round and alerted police this morning.

Police put a cordon in place around the property today and evacuated three nearby houses as a precaution.

The Defence Force disposal unit blew up the projectile in a controlled explosion at Winstone Quarry. They place the item under approximately a metre of gravel in a safe part of the quarry and detonated it by remote from 300 metres away.

A quarry employee says the bang was disappointingly quiet.