Three people have been killed and at least seven injured 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 George's Road in Hobsonville.

It is understood another person was also killed at the site where a new high school is being built.

A shaken construction worker Sam Nuthall, who was working at the 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 and there were a few "pretty bad" injuries after the tornado struck at about 12.15pm, he said.

The Hobsonville Point secondary school, due to open in 2014, and Hawkins Construction spokesman Hamish Anderson told NZ Newswire the company was still gathering information on the disaster.

He couldn't confirm the deaths.

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.

The Fire Service spokesman said there were reports of people trapped in buildings.

An ambulance communications spokeswoman said it was dealing with seven injured people.

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

Auckland Civil Defence is monitoring the incident and some bus services have been stopped.

State Highway 18 is closed as have a number of roads in the area. says another band of thunderstorms and torrential downpours are moving back to the area.

"We are advising people to remain indoors until the torrential rain and possible thunder has passed, most likely over the next hour or two," says head weather analyst Philip Duncan.

A thunderstorm warning is also in place for Waikato.

NZ Newswire

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