Northland, Coromandel Peninsula, Mt Taranaki, Bay of Plenty, Rotorua, the ranges of Gisborne north of Ruatoria, Tasman west of Motueka, Buller and Westland are under heavy rain warnings and watches.
MetService has also added a strong wind watch for South Taranaki, inland Whanganui and Taihape tomorrow and a heavy snow warning for the Canterbury high country.
They also warn that areas that were saturated with the midweek rain could see slips and flooding during this deluge and to stay vigilant.
A number of road snow warnings are also in place for alpine passes in the South Island, with forecasters warning of "impactful snow" across inland regions.
"With the upcoming school holidays it could be worth taking the weather into account for your travel plans," warned MetService.
Forecasters also warn to get used to this weather, as this is the first of many major storms that will sweep across the country over the next fortnight.
Niwa weather said a series of strong lows, fuelled by rain from the tropics, was expected over the next two weeks.
"The theme will be wetter than normal conditions with the potential for flooding," said the forecaster.
Along with heavy rain approaching warning criteria, Northland is likely to be deluged with thunderstorms, according to MetService's severe weather outlook.
The large low and associated fronts were expected to move onto New Zealand from the west tomorrow, bringing heavy rain to many areas and strong winds to the north and snow to the central South Island.
Another was set to hit on Monday and more later in the first week of the holidays.
Weatherwatch.co.nz said the North Island would bear the brunt of the most unsettled weather in half a decade, with at least six lows bringing bursts of rain and wind in the next 10 days.
The forecaster said although not all lows were stormy, at least three were expected to produce "severe weather risks".
This winter pattern was the most unsettled the North Island had seen in half a decade, after five years of general rain shortage across the top half of the island, the forecaster said.