The strong wind gusts which tore through Devonport last night were caused by a sudden collapse of air from the sky, like someone "pulling a plug'' on the storm, MetService says.
Winds of up to 110km/h hit the Auckland suburb shortly after 6pm, ripping up fences and sheds, toppling trees and powerlines and leaving hundreds without electricity.
Initial reports suggested a tornado, but forecasters today classified the wind as "strong, straight line wind gusts caused by a downburst''.
MetService meteorologist Dan Corbett said a thin band of thunderstorms, which moved in from the Tasman yesterday afternoon, hit a front of cold air and collapsed, causing the downburst.
"It's almost like somebody pulled the plug,'' he said. "It collapsed and there's a big rush down to the surface. As the cold air hits the surface it spreads out, it's called a downburst, you get a big rush of wind in all directions, and we saw that in Devonport last night.''
The downbursts are "very localised", he said.
It was the same weather phenomenon that caused the deaths of three workmen in a storm in Hobsonville last December.
Mr Corbett labelled thunderstorms "big wind machines", which pick up warm, moist air as they grow. When they hit cold air tornadoes and downbursts can occur.
In a thunderstorm which develops a tornado the storm clouds have stronger "upper level dynamics'' which fight against the cold air and rotate to form a twister.
"The thunderstorm yesterday didn't have the upper level dynamics, or the oomph, to generate that,'' Mr Corbett said.
However, every thunderstorm has the potential to generate such downbursts, given the right conditions.
"It does depend on the nature of the atmosphere, the nature of the storm. Every storm is different,'' he said.
While forecasters can predict a storm and which direction it might move in, it is difficult to predict the nuances in the atmosphere which might cause a downburst, Mr Corbett said.
But he said there's "always the risk'' of damaging wind gusts during a thunderstorm.
He advised people to check the MetService website for the twice-daily thunderstorm outlook, which will also provide an indicator of how severe a thunderstorm may be and whether there will be a chance of high winds.
If a downburst occurs the advice is to stay inside and keep away from windows.
The wind struck just after 6pm yesterday, with most damage confined to the area east of the Waitemata Golf Club.
Debris littered Vauxhall and Wairoa Rds in Devonport, with large sheets of corrugated iron hung on power lines like washing hung out to dry.
A handful of properties remained without power this evening, down from 600 at the height of the event.
Contractors were today continuing to clear loose debris while tarpaulins were used to cover some properties damaged by the wind.
Civil Defence controller Clive Manly said building inspectors were assessing properties damaged by the squall.
Air New Zealand cancelled several flights to provincial centres as the wild weather moved down the country.
