Three people are dead and seven are being treated in hospital after a tornado hit Auckland shortly after midday.

A Fire Service communications spokesman told NZ Newswire two were killed when concrete slab landed on a truck on St Georges Road in Hobsonville.

Civil Defence says three people have been confirmed dead, but Superintendent Bill Searle told media two were dead and they held grave concerns for another construction worker at the site where a new high school is being built.

The site was still being cleared of debris.

Seven people were in hospital, though none were regarded as seriously injured.

Three men were taken to Auckland Hospital with respective head, chest and trauma injuries and four people were being treated at North Shore Hospital. One woman had pelvic injuries and three others had cuts and abrasions.

Around 150 homes have been damaged and residents are being housed at Whenuapai air force base.

Power has been cut to around 400 customers in the Hobsonville-Whenuapai area after trees fell on lines.

Prime Minister John Key whose electorate covers the damaged area extended his sympathy to friends and family of the victims.

Shaken construction worker Sam Nuthall, who was working at the school site, said he saw big dark clouds and thought it would just be a rainstorm, but was then forced to shelter between a block wall and a truck.

"You couldn't see it coming ... there was debris flying everywhere, steel getting thrown around," he told Radio New Zealand.

There were 50 to 80 people on the site after the tornado struck at about 12.15pm, he said.

Hawkins Construction spokesman Hamish Anderson told NZ Newswire the company was still gathering information on the disaster.

A Whenuapai resident Suzanne McFadden told 3News it was "five minutes of utter devastation".

"It got very muggy, very still. Suddenly the sky went very, very black, almost like night, and a huge wind came across the river.

"It ripped century-old pine trees out of the ground, it's taken roofs off houses, it's crushed cars and boats.

"The main street of our village looks like a bomb's gone off and we're all very aware that it could turn around and another one could strike," she said.

Police have set up an operations base at Whenuapai and are advising motorists to be wary of flooding and drive to the conditions.

Some flights in and out of Auckland Airport had been delayed or cancelled and some local bus services stopped.

State Highway 18 is partially closed and some roads in the area are also blocked.

Loretta Radich arrived at her business on Hobsonville Road just as the storm hit.

"We saw the rain going in all different directions and the windows were flapping," she told NZ Newswire.

Government forecaster MetService is warning of the likelihood of possible tornadoes and thunderstorms as the weather front heads south to the Bay of Plenty.

NZ Newswire

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