The Army has detonated a live World War II bomb after a family found it on their property at Kinohaku.

Te Awamutu constable Rainier Bron said a man notified police on Wednesday, June 6, about a mortar shell on his family's farm.

The family had owned the bomb for many years and as children had played with it on their grandfather's farm, he said.

"The grandfather used to be part of the Home Guard during World War II. They think that's where the mortar shell had come from."

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"They never thought it was live and they lost track of it about 20 years ago but recently found it again."

The man was planning to dispose of the bomb himself.

"He put the old mortar shell in his car boot and drove north looking for a place to bury the bomb.

"But as he was driving along he thought he should get it checked out. I think it had been playing on his mind."

The man stopped in Pirongia, still with the bomb in his boot, to contact police.

Police drove to Pirongia to check out the bomb and immediately called the bomb disposal squad.

"They had a look at it through Facetime and came down pretty quickly from Auckland."

While police waited they drove the vehicle to an empty paddock at Pirongia Rugby Sports Club.

"We parked the car away from houses and waited for the army to come down."

The bomb was stored overnight in the Te Awamutu Central Police Station and the next day was detonated at a farm south of Kihikihi.