A slow-moving subtropical low is bringing hours of steady rain to Northland and is set to make a wet weekend for much of the country.

The area is under a severe weather warning, as the heavy rain could cause streams and rivers to rise rapidly. There's an increased risk of surface flooding and slips. Thundery downpours are also possible before mid-afternoon.

The downpour comes ahead of a potential cyclone developing around the Pacific Islands, near Samoa, which forecasters say could directly hit New Zealand.

Northpower has reported power cuts in Maungakaramea, Maungatapere, Tauraroa and Waiotira, although it is not clear if the outage is weather-related.


Weather stations north of Whangarei have recorded 30-35mm in the past six hours, Metservice meteorologist Nick Zachar said, and about 35mm fell in the wettest areas overnight.

"The main mass of rain is moving slowly to the south but it's still solidly over Northland," Zachar said. "It's not dumping down, but there's still a risk with steady rain of some surface flooding."

Another 30-50mm of rain could fall in isolated parts of Northland south of Kerikeri this afternoon, Zachar said. The heaviest rain would reach Auckland "in the next few hours".

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The widespread rain is caused by a warm front bringing moisture-laden air down from the tropics. A second front is not far behind and will bring a less intense rain band to the upper North Island this evening and overnight.

Much of the North Island will get rain by tonight but exactly where is still unclear, Zachar said. By tomorrow evening rain is expected in Wellington and the upper South Island including the Nelson ranges and Marlborough.

Auckland and the Coromandel are under a severe weather watch until 3pm, and the East Coast north of Tolaga Bay through to midday, with potential for heavy falls.

The downpour comes ahead of a potential tropical cyclone developing around the Pacific Islands, near Samoa, which forecasters say could directly hit New Zealand.

WeatherWatch.co.nz said the low pressure system would influence the northern North Island for several days.

"The peak chance for rain in the South Island will be from Saturday night to Monday morning, while in the North Island it may linger well into next week.

"There may be some isolated thunderstorms with downpours too."

However the lower South Island could miss out on the worst of the rain.

The band of rain is very narrow - meaning if it moves a little west or east the chance of rain could decrease to a drizzle or even dry spells, WeatherWatch said.