Weather: Why Auckland was hit so hard by storm

Devastating cyclone-force winds howled over Auckland on Tuesday night, catching an unprepared public by surprise.

So why were the winds so strong? head forecaster Philip Duncan says there were a number of moving parts.

"We had a deepening low moving in from the Tasman Sea - we then had that low making landfall in Waikato and brushing south Auckland, which then placed the strongest winds directly over Auckland City itself," he says.

"To help keep the energy, the powerful nationwide Antarctic southerly was roaring up behind it fuelling more winds.

"Finally, on top of all that, these worst winds then tracked across Auckland via a natural wind tunnel. All of that combined created damaging gales that were a notch above the forecasts".

Duncan Garner has slammed Auckland officials for failing to be prepared for the storm.

"I reckon that the authorities in Auckland this morning were as disorganised as f**k," the AM Show host said on Wednesday morning.

"There was no warning - no official warning from NZTA, Civil Defence... I didn't get a clear signal from any of these authorities that there was a problem."

However NIWA forecaster Chris Brandolino says we "got off lucky".

"If this happened at four in the afternoon, or six in the afternoon when people are out commuting, tree limbs are falling on commuters, people going over the bridge," he told the AM Show.

"It could have been a lot worse - as bad as it was, let's put that in perspective."

Weather Watch's Mr Duncan says the area of damaging winds was relatively small but just happened to be placed over our largest city with 1.5 million people.

"Waikato to the south wasn't very badly affected and neither was Northland just up the road. So it was really the wind tunnel effect through Auckland coupled with a deepening low making landfall that helped ramp up these gales even further," he says. says the low has weakened and the worst winds have passed, but strong gusts from the south will continue today.