Police are warning Cromwell residents near Waenga Drive to stay inside after a ''spate'' of mini-tornadoes left a trail of damage, throwing objects up to 500 metres.

Sergeant Bruce Terry of Cromwell said the mini-tornadoes had thrown loose building materials, such as roofing iron and even a fence, up to 500m.

A trampoline had been thrown by the wind and a few windows had also been blown in.

No injuries have been reported so far, but police were warning people to stay inside.

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A police spokeswoman said it was thought there had been three or four mini-tornadoes.

Cromwell Transport driver Dylan Tredinnick said he was loading a truck in the yard on McNulty Rd after 5.30pm when he saw the ''mini-tornado'' in the distance.

He saw building paper and iron flying in the air.

"I was just in my truck and I looked across and it sort of came out of nowhere.''

The company's manager, Jason Scott, who went out to survey the damage, said the mini-tornado had torn roofing iron of one house and building paper off some homes being built on nearby Stowell Dr.

It was not windy, he said.

Police said motorcyclists and drivers of heavy vehicles should avoid travel through the Cromwell basin, and warned there may be delays as power poles have come down in some places.

A Fire Service spokesman said firefighters from Cromwell Station were called to a powerline down on the corner of Ord and Kawarau Gorge Rds in Cromwell at 5.49pm.

Firefighters were assisting police with traffic control, he said.