A storm blasting Antarctic weather over New Zealand is currently the biggest storm on earth according to WeatherWatch.
The low stretches from just south of Fiji to Antarctica's ice shelf, but only 20 per cent of the storm is affecting New Zealand.
WeatherWatch.co.nz said the forecast air pressure at its centre over the next 24 hours would be greater than that of Hurricane Katrina when it made landfall in 2005.
Weather expert Philip Duncan said, "It's fairly normal to get a cold snap in late May. We're less than a week away from winter, it's not surprising to get a blast like this."
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But he said the low's strength was surprising.
"The depth of this storm south of New Zealand is up there with some of the biggest hurricanes we've seen. It's a really big storm."
However, WeatherWatch.co.nz said only the storm's edges were affecting New Zealand - its centre lay around 1000 to 1500kms to our south east.
Southland, Otago and Banks Peninsula woke this morning to the heaviest dumping of snow so far this year.
Overnight, areas of Central Otago and Southland saw up to 30 cm of snow, according to Metservice.
The North Island didn't escape the cold weather with light snow falls reported in Napier and heavier falls on the Napier-Taupo Rd.
Queenstown Airport was temporarily closed, with heavy snow right down to the township, and snow flurries blanketed Invercargill.
Dunedin was battered by an onslaught of nasty weather, said MetService, hit by snow, showers and offshore thunderstorms.
15 cm of snow was reported just above the city, while its hill suburbs saw around 10 cm.
Mr Duncan said the South Island low had joined forces with the subtropical low that affected the North Island on Saturday.
"The two of them are working together to dredge up Antarctic air.
"So we've got this air at the moment being pulled up from Antarctica, and dumped over New Zealand."
There were a number of road closures for eastern parts of the South Island, with drivers advised to use extreme caution and to drive to the conditions.
MetService said while it was a cold morning, nowhere reached a record low for May.
Winter's icy grip took hold in the North Island as well, with snow reported down to sea level in Hawke's Bay.
Light snow falls were reported in the Napier suburbs of Taradale and Maraenui, and motorists experienced sleet on the way to work between Hastings and Napier.
Heavier falls were experienced inland, on the Napier-Taupo Rd and further north at Waikoau and Putere,
Cold southwest winds continued to bring showers across Auckland, and saw a severe weather warning put in place for Wairarapa and western Bay of Plenty where there was a risk of gales.
Last night's lightening storm and strong winds in Dunedin cut electricity to Musselburgh, Port Chalmers and part of the Otago Peninsula.
Aurora Energy said lightening damaged transformers, while severe winds brought down a power line.
Central Otago and Wanaka also experienced black outs.
Delta crews were responding to faults and making repairs as soon as it was possible to do so safely.
Overnight wind and rain and this morning's brief snow in Hawke's Bay resulted in power cuts in some "higher-up" rural areas.
Unison Networks customer relations manager Danny Gough said "a few hundred" customers had their supply cut, in some cases for up to two hours.
Temperatures were expected to rise slightly tomorrow, with warm weather set to return by Friday.
Meteorologist Georgina Griffiths said, "On Thursday morning, expect frosts across many parts of the North Island, as wind drops out, leaving the cold air behind."
The minimum temperature forecast for Thursday is 6C for Auckland, Tauranga and Wellington, 3C for Napier, 1C Palmerston North and 0C for Hamilton, Taupo and Masterton.
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Motorists are being warned to expect hazardous driving conditions.