In Hamilton the low would dip to -2C, a drop of 7C compared to last night, Mr Lee said.
In the South Island cold nights are old news but further north tonight might feel especially frosty.
"We've had warmish spell so it feels a bit colder but it's actually average [for this time of year]."
Down south, parts of the high country would reach lows of -4 to -5C and Christchurch could expect a low of -2C.
The cold snap in the North Island follows a record warm period this year, with the hottest first six months of the year recorded by NIWA since records began.
Overnight lows and highs for tomorrow:
Whangarei: low of 4C, high of 15C tomorrow.
Auckland: low of 4C, high of 14C tomorrow.
Hamilton: low of -2C, high of 13C tomorrow.
Rotorua: low of -1C, high of 11C tomorrow.
Wellington: low of 5C, high of 14C tomorrow.
Christchurch: low of -2C, high of 12C tomorrow.
Dunedin: low of -1C, high of 12C tomorrow.
Invercargill: low of 13C, high of 11C tomorrow.