A rusty but live grenade was found in a pile of dirt at a Richmond recycling centre on Sunday.

Patricia Murrow has seen people dump everything from engines to metal fence posts at her greenwaste recycling centre in Richmond, near Nelson, but the site of a rusted grenade sitting in a pile of dirt yesterday threw her.

An employee was scouring the dirt hills at the Green Waste to Zero centre pulling out weeds when he came across the grenade.

"It could have been there for years," Murrow said.

She immediately went out to look. The pin was rusted out and not wanting to take any chances she immediately called the police.

"I wanted to see if it was a toy."

Murrow marked the area with a upturned paint bucket and waited.

"I wasn't taking any chances."

Police cordoned the area off until the Defence Force bomb squad arrived from Christchurch.

They confirmed that the grenade was live and posed a serious risk.

It was not until about 9.20pm the grenade was detonated. The explosion was heard several kilometres away. All that is left is a small crater in the dirt.

"Thank heavens it is not an every day event," Murrow said.

The site was previously an old tip so the grenade might have been dumped there many years ago and been dredged up.

"It's amazing what turns up," Murrow said. "People are always putting things in greenwaste that they shouldn't.

Police would like to remind members of the public that all military ordnance should be treated as live and very dangerous. Anyone who locates any old bombs or munitions should not touch them and should call 111 immediately.