MetService warned motorists in these areas there could be black ice on some roads.
The cold did not quite reach the North Island due to some cloud cover and light winds.
Showers in Northland and Auckland, and up the east coast from Wellington to Gisborne, would clear through the morning for a fine day, Martens said.
"That will be the story across most of the country, with those showers easing in the north and clear skies in the south."
Make the most of it though, as another front would arrive tonight in Fiordland and Westland, spreading rain across the country tomorrow.
"It will be the same story - rain heaviest in western areas, dry in the east.
"This front will bring a few days of unsettled weather, until a weak ridge develops over the South Island on Friday."
While temperatures in the North Island were forecast to drop tonight, bringing frosts to central areas, the north/northwesterly flow over the coming days would bring warm air across the country.
Today's weather
Whangārei
Fine apart from some evening high cloud. Southwesterlies turning northerly. 15C high, 8C overnight.
Auckland
Fine apart from isolated morning showers and areas of high cloud in the evening. Southwesterlies turning northerly. 15C high, 6C overnight.
Hamilton
Mainly fine, some morning cloud. Southwesterlies dying out. 14C high, 1C overnight.
Tauranga
A fine day. Southwesterlies dying out. 15C high, 5C overnight.
New Plymouth Fine spells. Southwesterlies dying out, then northerlies in the evening. 13C high, 5C overnight.
Napier Mainly fine, chance early shower. Southwesterlies dying out. 14C high, 0C overnight.
Wellington A few showers, clearing morning and becoming fine. Southerlies dying out afternoon, northerlies developing evening. 11C high, 6C overnight.
Nelson Fine with morning frosts. Northerlies developing afternoon. 12C high, 4C overnight
Christchurch Fine with morning frosts. Northeasterly developing afternoon. 11C high, 3C overnight.
Dunedin Fine with morning frosts, then high cloud from evening and northerly picking up. 10C high, 6C overnight.