The polar blast which roared across New Zealand was the most powerful storm in almost a decade.

The event was the right ingredients coming together, said MetService spokesman Daniel Corbett. "On Wednesday there was a surface low just to the northwest of the South Island and at the same time there was a southerly coming in with some cold air at the top of the atmosphere.

"The whole thing developed as a massive low that started to move across the west of the South Island, then [on Thursday] it moved across Cook Strait to create a new low just east of Kaikoura ... that was the real kick in pants that caused a clockwise circle of wind."

Associate Professor James Renwick at the School of Geography, Environment and Earth Sciences at Victoria University has researched how polar conditions affect our weather.


"To get an event like this, which is pretty extreme, we need the westerly wind that normally blows across New Zealand and the southern oceans to slow down and to buckle into a series of big meanders, north-south waves around the hemisphere."

The air was so cold over the country, temperatures dropped to -6C in Tekapo, because the isobars stretched all the way to down to to the edge of the Antarctic sea ice , said Associate Professor Renwick.

NIWA said the storm was an extreme event but not as intense as the Wahine storm of 1968. In February 2004 three people drowned after a storm caused widespread flooding and forced thousands from their homes.