A "big punch of cold air" is about to hit New Zealand, with windchill temperatures set to drop to as cold as -20C in the coming days.
NIWA principal scientist Chris Brandolino says the "brutally, brutally cold" temperatures are coming from Antarctica, "right from the ice".
"It's coming right from deep in the polar region," Brandolino told The AM Show on Monday.
"The South Island's going to cop the worst of it."
Although ski fields in the South Island are set to get a good covering of snow, the forecast has brought a warning for farmers to protect their stock.
"The windchills are going to be brutally, brutally cold. By sunset tonight windchill for the South Island, the lower third of the South Island, [will be] -10C, -5C, -15C.
"This time tomorrow for the South Island, the windchill factor - the feel temperature - is going to be -15C, -10C, maybe -20C," Brandolino said.
Strong winds are also forecast on Monday afternoon for coastal Southland, while Wellington will have wind gusts of up to 95km/h on Tuesday morning. The capital will also have a windchill of -5C.
According to WeatherWatch, the air will "progressively get colder" on Monday in the southern part of the country before "marching northwards over the rest of New Zealand tonight and across Tuesday".
"The colder the air is, the drier the air is - so snow and rainfall accumulation isn't overly excessive for most but clearly Southland and Otago will have the heaviest snow falls, above 100 or 200m (and heaviest above 400m) with 30 to 40cm possible. Central Plateau will get several centimetres of snow," WeatherWatch said on Monday.
Brandolino said Aucklanders can expect temperatures to drop to "near zero" on Wednesday, with frost likely.
The cold snap will last a few days, before next month brings more variable weather.
"We see more variability for the month of July - so a lot more ups and downs - but there's certainly not going to be any sustained cold weather," Brandolino said.