The shortest day of the year in the Southern Hemisphere is almost here and, temperature-wise, we are going to feel it.

Tomorrow night at 10.07pm is the Southern Hemisphere winter solstice, marking the day with the shortest period of daylight and longest night of the year.

It occurs when the sun's daily maximum elevation in the sky is at its lowest.

Given the length of New Zealand in terms of longitude, there is more than an hour difference in the amount of daylight different areas will experience.

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Invercargill will see just 8.35:01 hours, while Kerikeri will get 9.46:43.

As for the main centres, Christchurch will have 8.56:23 hours, Wellington 9.11:24 and Auckland 9.37:55.

The short day continues a rather bleak month for Christchurch, which by June 19 had only recorded 16.5 hours of sunshine, NIWA meteorologist Ben Noll said.

This included a six-day period, from June 10-15, when there was no sun at all.

The month's average was about 119 hours.

The main drivers of the pea-soup weather were fog and onshore winds, which brought moisture inland and trapped it between the mountains and the sea.

"Unless you have some westerly winds, that moisture gets trapped," Noll said.

"It was almost the perfect storm for not much sunshine."

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So far June had been relatively warm, especially in the northern and eastern areas of both islands, Noll said.

"The first three weeks of winter have been bearable, even subtropical at times.

"But as we pass the solstice, we are entering a colder pattern. On Saturday morning it will be felt from Invercargill to Cape Reinga. It will be a morning to wrap up in a blanket in front of the fire."

Temperatures in the upper North Island will drop into the low single digits overnight, with Auckland dropping below 5C, and potential frosts further south.

Waikato and the central plateau could drop below 0C.

In the South Island temperatures will range from -5C to 0C, even on the coast.

"It is going to be a fresh one," Noll said.

Temperatures will warm slightly on Sunday before a front on Monday and Tuesday ushers in an even sharper cold snap.

"The last few days of June and early July are going to be well and truly winter," Noll said.

"The silver lining, though, is it is all downhill from here."

The first few days past the solstice, the amount of daylight will increase by just a few tenths of a second.

However, by July and August the days would be noticeably longer.

Another silver lining was the cold snap was likely to bring snow to the southern skifields, Noll said.