Police in parts of the the South Island have been swamped with call outs to car crashes on icy roads.
Last night was the coldest of the year, with temperatures dropping to freezing level in Dunedin, Alexandra, Ashburton and Masterton.
Mount Cook airport recorded an overnight low of -11.3°C - the eighth-lowest temperature recorded since records began 24 years ago.
Police said they had been having trouble keeping people off the roads in those areas, and had dealt with 34 crashes this morning.
Jared Kirk from the Southern District Command Centre says two people were taken to hospital by helicopter.
In Queenstown a driver was arrested after they were clocked driving 180km/h on icy roads.
The 40-year-old man has had his licence suspended and will appear in the Queenstown District Court on Monday.
Police said there was also ice on footpaths so it would be dangerous for people walking or on bicycles.
They asked people to stay indoors until the weather improved.
Metservice meteorologist Tom Adams said more ice was expected tonight.
"It is going to be very cold in the South Island, so there will be frosts and there will be ice - albeit slightly whiter than the black ice we were talking about last night - but there still will be a lot of slippery patches on the roads."
Flooding in New Plymouth
Further north, New Plymouth has had minor flooding after being drenched by a short, sharp storm.
Metservice said the eye of a storm went directly over the city about 2pm.
Mr Adams said about 25mm of rain fell in an hour over the city, and up to 50mm within three hours offshore.
He said more rain could be on the way for New Plymouth, but it was unlikely to be as heavy as this afternoon's downpour.
The fire service said it had 10 call outs for minor flooding.