Temperatures plummeted in Christchurch this afternoon, dropping 10C in only 30 minutes, according to MetService.
It followed a night of muggy temperatures across the North Island, with Kaitāia taking top spot overnight temp at 20.7C.
Purerua and Auckland City were close second and third with 20.4C and 20.3C, respectively.
However, the forecaster said a "sharp drop" for Christchurch temperatures this afternoon was due to a "cool southerly change".
1News meteorologist Dan Corbett said in the hour from 1pm to 2pm, observed temperatures at Christchurch airport dropped by 11.3C from 28.5C to 17.2C.
It followed a "busy 36 hours" of weather across the country, including rain and potential thunderstorms.
"Still some rain to come for the northwest of the South Island, " said meteorologist Heather Keats this morning.
Heavy rain watches remained in effect for the Tasman District about and west of Motueka, Buller District, and the ranges of the Grey District into this evening.
"We do have another rain element to be mindful of today, and that's thunderstorms," said Keats.
"Anyone in the North Island can tell you just how muggy it is, and that humid air mass is streaming over northern and central New Zealand.
"This will contribute, along with daytime heating and wind convergence to trigger some heavy showers and maybe even thunderstorms."
She said mugginess would be replaced with a south-west flow over the weekend, which brought the "sharp drop" this afternoon.
"The North Island gets one more night of mugginess before it returns to more average temperatures tomorrow," she said.
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