Frosty mornings will continue over the next couple of days after temperatures plummeted well into the negatives overnight Sunday.
In the south, Christchurch dropped to -4C overnight and in the north, Rotorua fell as low as -1C.
WeatherWatch warns more severe frosts are expected across the South Island on Monday night.
"Peak winter means big frosts for the South Island with overnight lows tonight dipping down towards -10C through inland highland parts," the forecaster said in a statement.
"The -10C will be near places like Tekapo, Twizel and Arthur's Pass."
In addition, MetService says large parts of the country will see heavy rain or strong winds from Tuesday until Friday.
"Rain and wind associated with this system are forecast to affect parts of the North Island during Tuesday and Wednesday," the forecaster's severe weather outlook says.
"There is high confidence of warning amounts of rain and severe gale southeast winds in Northland during Tuesday morning and warnings for these are already in force.
There is moderate confidence of severe gale southeast winds in exposed parts of Auckland north of the Harbour Bridge, including Great Barrier Island and Coromandel Peninsula during Tuesday morning, and a strong wind watch is already in force for these areas."
MetService warns heavy rain could cause flooding in some areas.
"Expect a further 70 to 90mm of rain to accumulate on top of what has already fallen in the north and east of Northland, and 50 to 70mm elsewhere," it says.
"Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous."
On Monday and Tuesday, wind gusts of up to 100km/h could cause large swells on the North Island's East Coast, NIWA says.
Meanwhile, more "severe frosts" are forecast for the South Island overnight with Christchurch told to expect -2C and Queenstown -6C.
