A high-pressure system over New Zealand is likely to mean clear skies but could also lead to freezing temperatures.

NIWA says a record high pressure system is currently over New Zealand and it is set to strengthen on Thursday.

"Air pressure is the force exerted by the weight of air above a given point on the Earth's surface," NIWA says.

"It decreases with altitude and varies with weather conditions."

The high pressure system that is currently over the country is set to peak in intensity tonight and on Thursday, potentially breaking the national mean sea level record of 1046 hectopascals (hPa), NIWA says.

NIWA says in a high-pressure system air descendes and spreads outward "typically leading to clear skies, calm conditions, and stable weather".

Despite the lack of rain, it can cause frosty and / or foggy conditions, NIWA says.

"In parts of Central Otago like Alexandra and Clyde, a phenomenon called an inversion may cause the fog and freezing temperatures to last all day!"

MetService forecasts below zero temperatures for many parts of Canterbury and Otago on Thursday morning.

MetService meteorologist John Law says there will be a cold, frosty morning in the South but also in parts of the North Island such as Waikato and Taupo which is forecast to have an overnight low of -2C.

"So a very chilly old start first thing on Thursday but with plenty of clear sky around."

A cold night os forecast for parts of the South Island where the skies were clear with a low of -4C forecast for Mount Cook, he says.

-RNZ.