MetService was warning of a moderate risk of thunderstorms, wind gusts up to 90km/h and hail through Taranaki, and the west of Waitomo, Waikato, Auckland and Northland, from this afternoon.
However, the bottom of the South Island was already beginning to feel the brunt this morning.
The coldest temperature recorded this morning was -8.9C at Mt Hutt at 6am.
"Showers will be turning to snow, lowering to 200m in Southland and Otago, and 500m across the rest of the South Island," Martens said.
Several South Island roads have been closed already due to the snow, including SH7 Hanmer Springs turnoff to Springs Junction in Canterbury and SH94 Te Anau to Milford.
Road snow warnings were in place for most of the alpine passes, including Lewis Pass (SH7), Arthur's Pass (SH73), Haast Pass (SH6), Lindis Pass (SH8), Crown Range Road and Milford Road (SH94).
A heavy snow warning was in place for Fiordland to midnight tomorrow, and 6am Tuesday for Southland, Clutha and Otago, with heavy falls forecast above 400m.
Most areas were unlikely to make it out of the single digits temperature-wise, and overnight these would drop into the negatives.
There was also a moderate risk of thunderstorms with strong wind gusts to 110 km/h and hail for Fiordland and coastal Southland from late afternoon, and also in the south of Westland in the evening and at night.
The only areas to escape any of the severe weather today were from Canterbury up the east coast to Gisborne/Hawke's Bay, with northwesterlies and temperatures in the low to mid teens forecast.
The cooler air mass was forecast to start arriving in the North Island this afternoon through to tomorrow.
A road snow warning was in place for the Desert Rd (SH1) in the North Island, with snow forecast to begin later today from about 3pm.
Martens said snow was forecast in the North Island down to 700m tonight, and 500m Tuesday.
There would likely be frosts tonight in central areas, and more tomorrow night.
"Tuesday is going to be quite a cold night," Martens said.
"Most areas in the central North Island will be into the negatives."
The weather was forecast to ease by Thursday as a ridge began to form.
A low would pass to the east of the country Thursday, affecting the Chatham Islands but leaving mainland New Zealand relatively unscathed.
The low would have a further chilling effect, however, preventing the country from warming up.
"It is going to be a cold week - time to turn on the heaters," Martens said.
Today's weather
Whangārei
Showers, some heavy with hail. Brisk westerlies. 15C high, 8C overnight.
Auckland
Frequent showers, chance heavy afternoon, easing evening. Brisk southwesterlies. 14C high, 8C overnight.
Hamilton
Showers, some heavy with hail. Brisk westerlies. 13C high, 5C overnight.
Tauranga
Showers, clearing for a time this morning and evening. Northwesterlies, tend brisk westerly afternoon. 14C high, 5C overnight.
New Plymouth Showers, some heavy and possibly thundery. Strong westerlies. 13C high, 7C overnight.
Napier Cloudy periods, chance shower afternoon. Gusty northwesterlies. 16C high, 5C overnight.
Wellington Mainly fine, but chance brief shower this afternoon. Strong northwesterlies, easing afternoon. 12C high, 4C overnight.
Nelson Mainly fine with high cloud, but chance morning shower. Southwesterlies. 11C high, 2C overnight.
Christchurch Fine with high cloud, but chance shower from afternoon. Westerlies. 11C high, 0C overnight.
Dunedin A few showers, more frequent, with sleet and hail from evening, snow lowering to 200 metres. Westerlies, strong at times. 7C high, 3C overnight.
