The upper North Island is in for a soaking as a major low arrives from the north Tasman Sea, spoiling any outdoor fun planned for the final week of school holidays.

Northland looks set to bear the brunt of the moist easterly flow, with warning levels of rain and potential for surface flooding expected from late morning tomorrow.

Meanwhile, Auckland and its parched dam supplies will receive less, but still some, much-needed moisture, while the major weather factor at play would be gale-force easterly winds arriving tomorrow evening.

MetService meteorologist Lewis Ferris said while it was already pretty wet and cloudy in the upper North Island today, the major weather events were set to kick off tomorrow around midday.

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A heavy rain warning is in force for Northland from 11am on Wednesday to 6am on Thursday, with up to 100mm of rain forecast.

There would be an increasing risk of isolated thunderstorms through the day, with potential for localised downpours of up to 35mm/h.

The system was looking to track mainly to the east, meaning while Auckland would see rain developing and spreading south in the morning, it would be much less severe than the Far North.

Still, given the city's water supply crisis any rain would be welcome, with dams sitting at just 57.5 per cent as of Wednesday, compared to an historical average of 82.8 per cent.

Meanwhile, Aucklanders continue to respond positively to water-saving requests, with the current seven-day rolling average of 392 million litres well below the target of 409 million litres.

Northeasterly winds however, could rise to gale by the evening in Auckland - up to 90km/h in exposed places, and a severe wind watch is in place from 6pm on Wednesday to 9am on Thursday.

Further to the east, the Coromandel and Great Barrier Island are in for a dowsing, with a heavy rain watch in place from Wednesday evening to Thursday morning, with up to 90mm of rain expected in that period.

The system would then continue east over Thursday and Friday, with potential for warning-level amounts of rain about western Bay of Plenty, Gisborne and the ranges of Hawke's Bay.

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The bad news is that in its wake would be an unsettled east-to-northeast flow across central and northern New Zealand, with continuing periods of rain for most of the North Island over the weekend.

"Unfortunately the final weekend of the school holidays is looking pretty wet," Ferris said.

It also wasn't good news for northern skifields, as the northern origins of the system meant it would likely be too warm for snow.