Temperatures have plummeted overnight, Auckland experiencing its coldest night since the height of winter last year.
The mercury dropped below -6degC in Twizel, while Hamilton and Christchurch also fell below zero.
The North Island's coldest spot was Waiouru, which shivered through a -5degC night.
NIWA forecaster Chris Brandolino told the AM Show on Monday it was Auckland's coldest night since August 2016.
"I'm sure the rest of the country south of the Bombays is embracing that and feeling our pain."
It's unusual but not unheard of for the tail end of autumn to bring cold snaps, he said, and the next few weeks - as winter proper begins - shouldn't be so harsh. Nor should winter itself.
"Our outlook, which covers the first two months of winter… for the months of June and July, including May of course, we think temperatures are likely to be average to above average for much of the country."
It's hard to say if climate change is to blame, however. While the Earth overall is warming, regional fluctuations can easily cancel out any warming in the short-term.
Mr Brandolino says the effect of climate change is more likely to be seen in rainfall patterns, than on your thermometer.
"You take place A in New Zealand, that may have the same rainfall or precipitation over a year, but how it's distributed will be more in bunches. If a place averages 500mm a year for rainfall, instead of being more evenly distributed, it may come in more bunches."
More heat in the atmosphere means more energy, which results in more extreme weather events.
"There is a relationship between a 1degC increase in the temperature of the Earth, and an 8 percent increase in extreme rainfall. An event that may produce 100mm of rain today will produce more in the future."
Even with nice weather forecast across New Zealand on Monday, nowhere will see the mercury rise above 15degC - Whanganui, Gisborne and Hawke's Bay topping the list.
Auckland and Hamilton should reach 13degC, while Queenstown and Dunedin will be wrapping up in the 10degC chill.