Weather: Bitterly cold morning for New Zealand as temperatures plummet below zero

It's been a freezing start to the morning on Tuesday as temperatures plummet around New Zealand.

It was the coldest night of the year down south, as South Islanders huddled inside with their fires and hot-water bottles.

"Ranfurly and Middlemarch (Central Otago) tied for New Zealand's coldest temp of 2018 (-6.0degC), which was the coldest temp since Sept 2017 for the country as a whole," NIWA says.

And as frosts carpet the country, the cold weather set a new record in Whangarei.

"With a min temp of 0.8degC in Whangarei this morning, it was the 3rd coldest May temp on record!" NIWA says.

"This was the coldest May morning since 1976 or in 42 years."

The rest of New Zealand didn't fare any better, with single-digit minimum temperatures countrywide.

"Sub-zero temperatures will be widespread in the South Island on Tuesday and Wednesday early morning with lows mainly between -3degC and -6degC inland," Weather Watch warns.

"The risk of frost and the soil freezing is high in most areas in the South Island and around Central Plateau in the North Island."

MetService says the chill is due to the fine and dry weather - which is expected to continue for the rest of the week.

"At night, the earth quickly begins to cool and the cold ground cools the lowest layer of the atmosphere in contact with it," it says.

"Under clear skies this outgoing heat is lost out into space but a layer of cloud overnight acts like a blanket, helping to trap the heat of the day.

"Clear skies and light wind make the perfect conditions for the temperature to really drop away overnight so be prepared for some cold and frosty mornings."