The polar jet stream is unleashing a sub-zero freeze over New Zealand this week.
NIWA warns "very cold air" from the Antarctic ice sheet is sweeping the country - and an even stronger shot of cool weather is coming on Thursday-Friday.
"The polar jet stream has been a key driver of New Zealand's weather patterns lately and that's set to continue - characteristic of a waning El Niño," the forecaster said of this week's conditions.
"In fact, this week's cold snap will reach as far north as Vanuatu and Fiji!"
The chill would ease in the middle of the month but could return later in May, NIWA added.
MetService said Kiwis should bundle up for chilly mornings - especially on Wednesday in the North Island and Friday in most of the South Island and inland North Island.
"Keep an eye out for brisk southerly winds in the lower and eastern regions of the South Island and North Island as the cold fronts move through your area this week," a post on the forecaster's Facebook page said.
"These may also generate large waves along these coastlines towards the end of the week."
Temperature-wise, Taupō and Christchurch were expected to plummet to -2C and -4C respectively on Thursday, while forecasters were anticipating 0C in Hamilton on that day.
Meanwhile snow flurries were possible for parts of Southland, Otago, Canterbury and Marlborough from Wednesday afternoon until early on Thursday. Snow as low as 500m was possible for a time in some areas.
Road snowfall warnings were in place for the South Island's Arthur's and Porters passes - valid from 6pm on Wednesday until at least 3am on Thursday.
The significantly lower temperatures marked "a stark contrast" to this time in 2023, MetService meteorologist Mmathapelo Makgabutlane said.
Last year, meteorologists said New Zealand recorded its warmest May on record - largely blamed on climate change.
