"Autumn has finally woken up," WeatherWatch says, ahead of a flurry of cold blasts which are likely to cause a 10C temperature drop across the country.
The forecaster warns a cold change will cause the notable temperature shift this week, bringing an end to a series of hot, humid days thanks to airflow from the sub-tropics.
An initial cold blast will arrive in the east of the country before Thursday, taking temperatures from above normal to several degrees below by Wednesday. In most centres, the change will see temperatures fall by more than 10C.
"The cold change moves into Dunedin today, Christchurch tonight, Wellington on Wednesday morning and Auckland by Wednesday evening," WeatherWatch said in a press release on Tuesday.
"The upper half of the North Island won't necessarily have a cold front moving in with rain then a temperature drop; instead the colder air will likely become more apparent later into Wednesday and overnight into Thursday morning.
"The upper North Island will again be warmer than average tonight while the South Island goes the opposite direction under the new colder airflow."
The cold change will perhaps be most noticeable in Kaikoura, where a NIWA Weather graphic shows a fall from 26.2C to 8.7C in just 19 hours.
There will be some rainfall in the west, the forecaster adds, with some snow across the South Island's ranges and mountains and lower and central parts of the North Island.
WeatherWatch warns this first cold change will be followed by another one or two this weekend. It says the rough weather pattern is normal for this time of year - and if anything, has taken a month longer to kick in than usual.
