Twister: The sky went black then mayhem

The sky was grey and damp in the morning, and that was supposed to be about the worst of it. In Auckland, the forecast was for "periods of rain ... clearing in the afternoon" with "freshening" northwesterlies.

At 9am, the MetService warnings concerned "frizzy hair syndrome" and not much else.

Then suddenly, about noon, the sky in the city's northwest went black - and the air "electric".

"I saw it coming across the river," said Suzanne McFadden, a resident in Whenuapai. "This was like a juggernaut roaring through here. Everyone is really shaken up."

In an instant, the calm was shattered. The wind whistled then roared as a tornado ripped through trees, roads, houses, power lines, garages, cars and boats.

"It was just crazy. There were trees everywhere ... there were windows breaking," a resident said.

A mother tried to console her young son. Then a trampoline crashed against her window.

"I couldn't see anything - it was just... white."

The twister tore off a concrete wall and crushed three workers at a construction site.

Debbie Booty heard a door slam then saw debris flying. Her lounge window exploded, showering glass everywhere. She ran into her closet.

"I shut myself in there and called my husband. I was panicking, I was terrified," Ms Booty said.

When she emerged about five minutes later she found water flooded through the house. The chimney was on a lean - like many in Hobsonville, almost torn off the roof.

The tornado's beginning and end were elusive; it melded into a thunderstorm wreaking wider destruction through Hobsonville and Whenuapai.

Waimarie Rd, Kay Cres, Wallingford Way and Stratton Pl were among the streets blasted with wind and water. Fields and highways flooded.

Hobsonville resident Jeff Pilling said his trampoline had been flung 50m. Outside his house was a scattered mess of corrugated iron, debris and a mangled portaloo.

The damage was littered through suburbs within minutes, but the tension continued for the next couple of hours as the storm hovered.

Emergency services sprang into action. The Fire Service sent a dozen trucks into the affected areas as the frequency of its logged incidents surged 2000 per cent. Firefighters responded to as many as 100 calls in the first two hours.

"There are houses with roofs off scattered all over West Auckland," said a spokesman amid the chaos.

Police assembled a makeshift headquarters at Whenuapai Air Force Base, while ambulances sped seven injured victims to hospital.

"There might be some critical," said a St John spokesman.

Debris and fallen trees lined flooded streets as residents, some of them in tears, assessed the damage.

Houses were crushed by trees; some completely flattened. Power lines had been ripped out, windows smashed and trees toppled. Entire roofs had blown away.

Several roads were closed, including State Highway 18.

Soldiers and sniffer dogs went in and out of houses searching for anyone who might be trapped. Neighbours knocked on one another's doors, while parents anxiously waited for news from schools.

"We live in a little village, and you just don't expect this kind of thing to happen," said Ms McFadden.

Authorities quickly established about 150 houses had been seriously damaged, and 250 residents were told to evacuate to the air force base.

MetService made no effort to explain the tornado during the afternoon, doubting its existence till the end. Spokesman Dan Corbett brushed such twisters off as "small things".

"We're not completely discounting it," Mr Corbett said, though the state forecaster had seen thunderstorms on its Doppler radar.

Meanwhile, photos streamed out from West Auckland of a shattered cityscape darkened by blasts of wind and rain.

As power was cut to several areas, lines company Vector issued a call that people should assume downed lines were live and to be careful.

Auckland Airport ordered staff off its tarmac to take shelter. At least 19 flight arrivals and 14 departures were cancelled amid high winds.

In the disaster area, Police Superintendent Bill Searle held a press conference at the air force base.

He confirmed there had been deaths and said about 12 uniformed Urban Search and Rescue (USAR) personnel, who happened to be in the area at the time, were gathering intelligence to help police. The Defence Force was on standby.

He said a series of tornadoes had passed through Hobsonville, and one touched down on a subdivision, causing damage to trees and buildings.

"We'd ask that people that don't have to be in the area stay away from the area," Mr Searle said.

"We're very keen to talk to the family members of anyone that might be injured," he said.

"If they make themselves known to the police officers we'll give them the necessary care and attention."

Auckland Mayor Len Brown was also there to be with Civil Defence operations.

"This is a real tragedy," Mr Brown said.

"Lives have been lost, people injured and property badly damaged. Those affected and their families are in our hearts and minds."

MetService issued a severe weather warning for Auckland during the storm, and lifted it by 3pm, marking its final passing.

SH18 was opened in time for evening commuters, though traffic in many areas still became backed up on motorways and wet roads.

Dr James Renwick, Associate Professor of Physical Geography at Victoria University, said tornadoes did strike randomly.

"Analysis of weather records does not show a pattern, nor are there trends obvious in tornado occurrences," Dr Renwick said.

"These events strike at random from time to time, but they are very localised and sporadic and are not obviously tied to trends in the large-scale climate. At this stage, we have no indication that tornado occurrences will become more or less frequent in future."

Philip Duncan, of Weatherwatch.co.nz, noted how close yesterday's tornado was to Albany, hit last year by another deadly tornado.

"There is clearly something about the geography of northwest Auckland that favours tornado development."

Both events had struck on relatively calm days, he said.

"But a very active and dramatic front passed through and the instability led to this freak event."

North Shore Hospital admitted four people who were injured in the tornado.

Three of them were expected to be discharged from North Shore last night, after being treated for minor injuries including cuts and abrasions.

A fourth female patient suffered a fractured hip

and would spend at least one night in hospital. She was in a comfortable condition.

No details were available about the patients.

A further three people were taken by ambulance to Auckland City Hospital.

They were treated in the emergency department and all were in a stable condition last night.

History of twisters

1981: A tornado rips through Rothesay Bay and Browns Bay, causing widespread property damage.

1991: A large tornado strikes Albany, destroying a church and killing one person.

2003: A series of mini-twisters hits Browns Bay, ripping tiles and television aerials off roofs and knocking over trees and fences.

May 3, 2011: A large tornado strikes Albany again, killing a construction worker, injuring 14 and causing widespread damage across the North Shore.

- Michael Dickison of the New Zealand Herald