THREE people are dead, seven are in hospital and about 150 houses are so badly damaged they are uninhabitable after a tornado cut a swathe through west Auckland shortly after midday.
Two people were killed when concrete slab landed on a truck on St Georges Road in Hobsonville, a Fire Service communications spokesman told AAP.
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, he said.
"It's early on in the investigation and we need to make sure we get all the facts straight before we release any information (on the two deceased and missing person)."
The death toll equals the previous worst tornado, which killed three people in Hamilton in 1948 and wrecked 150 houses.
Supt Searle said the tornado trapped a small number of people in their homes, but they had since been evacuated.
Police had been knocking on doors in the area and all residents had been accounted for, he said.
Families from up to 150 damaged homes - most home to Defence Force personnel - were evacuated and most of them will be housed at the Whenuapai Air Base.
"[It was] like a knife through butter for some of those trees," said mayor Len Brown. "A powerful tornado, devastating. Trees uprooted. Clearly a major weather event."
He said it was a "real disaster and tragedy for those families and for those that are in hospital and to the families who have been made homeless".
Power has been cut to around 400 customers in the Hobsonville-Whenuapai area after trees fell on lines.
Shaken construction worker Sam Nuthall, who was working at the school site, was 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.
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 were delayed or cancelled.
The weather front moved south to Bay of Plenty where another tornado touched down about 3.20pm at Hamurana injuring one person.
Trees and power lines were downed and roofs lifted in the town on the northern shore of Lake Rotorua about 17km from Rotorua.
Fire Service shift manager Colin Underdown said it also caused some children to be evacuated from Mokoia Intermediate, and an ambulance had to collect one person who was "thrown around by the tornado".
In May last year, a construction worker in Albany was killed when a tornado struck.
