Seven rain alerts are now in force across northern and western New Zealand as a tropically charged low sets to unleash a burst of wild weather.

Much of New Zealand will be cloaked under grey skies today, with rain starting to fall at the top of the country and worsening across the day.

The first of seven rain watches, that stretch from Northland to Westland, come into force at 11am, with rain not letting up until noon on Friday.

Gales are also set to batter much of the country, with MetService warning wild weather lies in store for nearly every region thanks to a sub-tropical rainmaker behind widespread flooding in New South Wales.

So far 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 watches.

MetService expects to add to the areas under warnings and watches today as the deepening low approaches the country.

It's the first several major storms that forecasters say will sweep across the country in the next fortnight.

In its severe weather outlook, MetService says as well as heavy rain approaching warning criteria, Northland would likely be rattled by thunderstorms.

The large low and associated fronts were expected to move onto New Zealand from the west during Friday, bringing heavy rain to many areas and strong winds to the north and snow to the central South Island.

Niwa weather said a series of strong lows, fuelled by rain from the tropics, were expected over the next two weeks.

Another was set to hit on Monday and more later in the first week of the holidays.

"The theme will be wetter than normal conditions with the potential for flooding," said the forecaster.

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, following five years of general rain shortage across the top half of the island, said the forecaster.