The Defence Force bomb disposal squad was called out yesterday after a live grenade was found at a recycle centre in Richmond, near Nelson.
Police said a staff member at the Richmond Green Waste Recycling Centre called police after they found the rusty grenade in a pile of dirt, about 1pm.
The area was cordoned off until the Defence Force bomb disposal squad arrived.
Police said the grenade was eventually detonated about 9.20pm, with the explosion heard several kilometres away.
Experts confirmed the grenade was live at the time and posed a serious risk.
Member of the public were reminded to treat any such finds as live and very dangerous, and anyone who located any old bombs or munitions should not touch them, and call 111 immediately.