Roads have closed at both ends of the country under a blanket of snow, as treacherous conditions sweep upwards from the South Island.
The Desert Rd in the central North Island and Lewis Pass, north of Christchurch, remain impassable with Metservice warning of more snowfall to come.
The forecaster also has road snow warnings in place for Arthur's Pass, Porters Pass, Lindis Pass, Crown Range Rd, Milford Rd, all within the Central Otago region.
Now, Weatherwatch warns Thursday could be the North Island's "coldest day yet".
"Hail showers to sea level are possible in both islands as the cold change sweeps through," head forecaster Philip Duncan says.
"Heavy rain and isolated thunderstorms will be possible and winds may also be strong in exposed coastal areas."
Metservice forecaster Tui McInnes says there is a chance of some hail over the Gisborne region, but conditions there have actually eased up since Wednesday.
Mr McInnes says last week's temperatures were extremely cold so it's too early to know how Thursday will compare.
"It could still warm up today," he says. "It's certainly not coldest we've experienced."
"But the system that's following wintry blast does mean temperatures could get a bit cooler, especially on Saturday... it could have some quite nippy temperatures."
The North Island's Napier-Taupo Rd already has a road snow warning in place, which will be reassessed at 11am on Thursday.
A "brief" heavy rain and snow severe weather watch was in place for northern Hawke's Bay and southern Gisborne overnight on Wednesday, MetService says. A heavy rain watch was also put over the Gisborne area, south of Te Puia Springs, "especially in the eastern hills".
In the Wairoa District a severe rain watch approached "warning criteria" on Wednesday night.
Metservice says a "ridge of high pressure" that is moving up the country will "settle conditions" on Friday.
"A front is forecast to move north over the south of the South Island on Saturday, and over the upper South Island late Sunday, bringing periods of rain to the western areas.
"The front then weakens over central New Zealand overnight Sunday, followed by another ridge over the country late Monday."
The New Zealand Transport Agency (NZTA) warns commuters there's a risk of black ice in Southland and Otago.
"Please slow down if you are driving on these highways in the early morning," NZTA senior journey manager Neil Walker says.
"Ice-prone highways will be gritted but snow or rain and freezing temperatures can still make them dangerous."