A Porirua police station was evacuated this morning when a man handed in a grenade he had found in his garden shed.

Senior Sergeant Martin Tunley told APNZ the grenade was handed in to the Waitangirua Station in Cannons Creek at 9.16am.

"It was placed on the ground and left."

The station and surrounding buildings were evacuated as a precautionary measure.

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The bomb disposal unit were able to see immediately that the grenade was inert, Mr Tunley said.

He said it was not known how the grenade came to be in the man's shed.

The cordon has now come down and police were back in their station.