Photo: UnSplash/ Aaron Burden
Cold air is moving up from a polar vortex around Antarctica.
NIWA says each winter, a ring of stormy, freezing weather - known as the polar vortex - encircles Antarctica.
Meteorologist Ben Noll said it typically kept harsh, wintry conditions locked up near the South Pole but on rare occasions it became disturbed and moved across New Zealand.
"Sometimes the polar stratosphere, a layer of atmosphere 10-20 kilometres above Antarctica, warms rapidly, with temperatures rising by more than 25 degrees Celsius in a week. This is called a sudden stratospheric warming and causes the polar vortex to weaken or become stretched and displaced."
Noll said it can influence the atmospheric layer and and the resulting weather.
"The icy air masses near the pole have an easier time escaping into the hemisphere's mid-latitudes, like an ice cube tray being shaken loose."
He said the phenomenon was currently being observed above Antarctica and would strongly influence the Southern Hemisphere's weather patterns throughout August.
It will contribute to a cold and frosty start to the month, followed by more moderate temperatures.
There have only been three sudden stratospheric warming's in the satellite era: September 2002, September 2010, and August-September 2019.
After the 2002 event, New Zealand experienced its coldest October in 20 years with below average temperatures covering much of the country and frequent ground frosts.
Noll said the weather-related impacts this year would mean cold, frosty conditions were likely to be followed by warmer and wetter conditions later in August and into September.
MetService meteorologist Lewis Ferris said many parts of the South Island experienced temperatures below zero on Friday morning.
"A very cold air mass coming up from the south has been bringing that potential for snow to lower parts of the South Island and bringing that cold air right across the country, especially around places that are still being affected by that southerly windflow."
The coldest place overnight was Mt Cook Aerodrome at -10.3 degrees Celsius, with Central Otago falling to -4C and Christchurch experiencing -1C.
Ferris said the cold temperatures were set to persist into the weekend - though most of the country would see fine, settled weather.
An overnight low of -5C in Christchurch meant for a frosty start to Saturday for those in the Garden City.
Ferris said most places would continue to experience cold mornings, followed by sunny afternoons.
He said the exception was Northland, which is under a heavy rain watch - with the potential for it to be upgraded to an orange warning from Saturday afternoon through to Sunday afternoon when the rain is expected to intensify and be more widespread.
"The wintry system that moved up the country this week has been a very nice top up for ski fields so it will make perfect conditions this weekend, but you will be dealing with larger crowds."