Temperatures have dropped below zero in places overnight as unseasonably cold conditions grip the country.

MetService meteorologist Karl Loots said the southerlies had been followed by a night of clear skies and light winds - the perfect recipe for cooling.

Waiouru in the central North Island and Tekapo in the South Island were the coldest spots at 5.30am, dropping to -0.5C.

Many places were well into the single digits, with Palmerston North, Taupō, Masterton, Queenstown and Upper Hutt all about 3C.

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Even parts of Auckland, normally kept temperate by the surrounding ocean, got to 8C in places - a far cry from the sweltering nights in the low 20s of late. Wellington also got to just 7C in the city.

"It is unusually cold for this time of year," Loots said.

Tuesday would provide a mild, fine and somewhat autumnal day, with temperatures in the low 20s for most.

Auckland would see a high of 23C with light winds.

Tonight another weak cold front would move on to the lower South Island, bringing periods of rain there and on the West Coast tomorrow. It would reach central New Zealand on Wednesday night and the North Island Thursday, but would have weakened greatly.

On Thursday and into Friday another ridge of high pressure would spread over New Zealand, persisting to the weekend and bringing calm and sunny days for most.

Loots said the calm conditions following the front could bring more cool nights, but they would likely be warmer than the frigid start to the week.

Today's weather

Whangārei

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Partly cloudy, chance shower until evening. Southeasterlies. 22C high, 14C overnight.

Auckland

Fine. Light winds, but afternoon sea breezes. 23C high, 13C overnight.

Hamilton

Fine. Light winds. 24C high, 8C overnight.

Tauranga

Fine. Southeast breezes, dying out at night. 23C high, 14C overnight.

New Plymouth Fine. Light winds, but afternoon westerly breezes. 21C high, 10C overnight.

Napier Cloudy periods and chance morning shower, then fine. Light winds first, northerlies developing afternoon. 22C high, 11C overnight.

Whanganui Fine. Light winds, but afternoon southwest breezes. 21C high, 11C overnight.

Wellington Fine. Northerlies strengthening. 18C high, 15C overnight.

Nelson Fine. Northerlies developing afternoon, changing southwest evening. 21C high, 12C overnight.

Christchurch A cold start to a fine day. Northeasterlies. 23C high, 11C overnight.

Dunedin High cloud. Northerlies. 22C high, 12C overnight.