An icy blast from the sub-antarctic is setting in, bringing bitterly cold temperatures to much of the North Island today.

Weather Watch analyst Philip Duncan said temperatures would be low enough for snow to fall to low levels in places such as Taupo and Rotorua, which was "very rare".

Duncan said the cold weather system had moved more slowly than expected and Aucklanders had woken to a spectacular day with temperatures reaching 15 degrees in the shade.

But by midday the cloud had begun thickening and winds had increased, with temperatures in the upper North Island predicted to fall several degrees in the next few hours.

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Wellington dropped to 3.7 degrees at lunchtime and Duncan said the southerly was making it even chillier.

"The wind chill, or 'feels like' temperature, is currently -5 in Wellington. In Christchurch the air temperature was just 2 at 12 noon with a wind chill of -6 degrees," he said.

Southerly gales have been gusting close to 120km/h in the capital.

Waiouru, in the central North island has zero degrees and snow flurries.

Duncan said snow could still fall as far north as the Hunua Ranges south of Auckland, and the northern tip of the Coromandel Peninsula.

MetService said strong and very cold southerlies were quickly spreading over the North Island and predicted snow to very low levels.

It has issued heavy snow warnings for Southland, Otago, Canterbury, Marlborough and the hills and ranges of Wellington, Wairarapa, Hawkes Bay, Gisborne, Manawatu, Wanganui, Taihape, Taupo and Taranaki.

MetService warned motorists to take care and farmers to shelter any vulnerable stock.

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The warnings were due to the low levels involved and snow amounts would probably be marginal, it said.

Heavy snowfall predicted for Canterbury has been fairly light and snow showers should clear by this evening in the South Island and tomorrow morning in the North Island, MetService said.

Duncan said holidaymakers in the north of the North Island could look forward to some respite on the final day of the long weekend.

"Strong south-southeast winds will clear a lot of cloud and showers away, so Monday might be a really sunny but cold day for northern and western New Zealand."

The rest of the week won't be much better for most, with a large high-pressure system "snaplocking" the frozen air and bringing severe frosts to both islands on Tuesday and Wednesday.

NIWA predicts another cold month ahead, with below-average temperatures likely in many regions.

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It also anticipates more frosts than usual for the South Island and central North Island.

- With NZHERALD STAFF, WeatherWatch