Weather: Chilly nights ahead for many as mercury plunges to 4C

Time to dig the winter pyjamas out of the drawer - overnight temperatures are set to plunge for many this week.

MetService meteorologist Clare O'Connor said "a big old high pressure system" was moving over Aotearoa, which would bring settled weather and clear skies for most of the country.

That meant warm days, with highs of 15-22 degrees for most, but the flipside was cold nights.

"Clear skies overnight means things can cool down easier, because when it's cloudy overnight it gives an insulating layer," she said.

"It's like having a nice duvet over the country - take that away and it gets a lot colder."

The chilliest nights for many would be Tuesday and Wednesday, O'Connor said.

Auckland Airport would see a low of 9C overnight on Tuesday, while Whenuapai would drop to 7C.

That was about 6C colder than what was normal for this time of year, she said.

In the central North Island - places like Hamilton, Rotorua, Masterton and Whanganui - the nights would be even colder, with lows of 4, 5 or 6C expected, she said.

Those were also about 6C lower than normal for mid-March.

The bottom of both the North and South Islands, as well as the east coast of both islands, were in for some showers and cloudy conditions this week, meaning they would not see such a drastic plunge in overnight temperatures, O'Connor said.

Slightly below average daytime highs were expected in those places, with "not much chance to warm up until the second half of the week".

MetService's extended forecast showed Wellington and Christchurch would only see highs of 17C during the first half of the week, but that would leap to 22C in Christchurch on Thursday.

Wednesday also marks the autumn equinox, meaning the nights will be longer than the days from then on.

Meanwhile, the National Institute of Water and Atmospheric Research (NIWA) said on social media that parts of New Zealand, including the upper and lower North Island and the upper and eastern South Island, would continue to experience dry soils.

"With below normal rainfall favoured in the next two weeks, soils will likely continue to get drier."

The government has classified the dry spell as a "medium-scale adverse event" in Marlborough, Tasman and Nelson districts, unlocking tax and other support for affected farmers and growers.