The lower half of the North Island kept above zero but only just: Taupo recorded 1C, Masterton 2.3C and Rotorua and Palmerston North are on 3C.
The warmest spot was in Northland, where Whangarei is on 8.2C.
MetService warned the country would be in the grip of hard frosts for the next couple of nights.
Although the South Island bore the brunt of the freezing temperatures last night, the rest of the country was expected to have their coldest night tonight.
MetService said the country could expect fine weather during the day and chilly temperatures at night as a ridge moved over the country this week.
Meteorologist Tom Adams said winds would be calm, the air cool and skies clear - therefore bringing overnight frosts.
"Temperatures rise on Wednesday, as northerlies pick up.
"But until cloud builds at the end of the week, it would still be a wise idea for gardeners to protect their spring seedlings and for motorists to be wary of ice on the roads.''
Today's forecast
Auckland: Fine, light winds. 15C
Hamilton: Fine, light winds, afternoon southwesterlies. 14C
Tauranga: Fine, afternoon showers. Light winds. 15C
Wellington:Showers, fine after lunch. Southerly dying out. 10C
Christchurch:Frosty, fine. Light winds. 13C
Dunedin:Frosty, fine. Light winds. 13C