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Auckland and Northland residents are being warned to prepare for "intense" thunderstorms and "damaging" gales up to 130km/h from this afternoon.
MetService has issued a severe weather warning, with the threat of gale-force winds at the top of the North Island until at least midday tomorrow. There are also "severe" gales forecast for Fiordland and southern Westland.
"Winds of this strength can damage unsecure structures, trees, powerlines, and make driving conditions hazardous," MetService said.
"This is a significant weather event for the country and people are advised to keep up to date with the latest forecasts."
MetService meteorologist John Law said there was a chance of hail from the thunderstorms likely to strike Auckland, Northland and the Coromandel throughout the day.
"It's looking very much unsettled, particularly around the northern parts of the country," he said.
Wind gusts of 110km/h in Auckland and Northland are possible from late morning with "damaging gusts" up to 130km/h expected later today and tomorrow morning.
"It's definitely a forecast to keep an eye on," Mr Law said.
"There could be some pretty intense thunderstorms, some fairly intense bursts of rainfall."
WeatherWatch.co.nz analyst Aaron Wilkinson said there was a low risk of a tornado forming during today's storms.
"In isolated thunderstorms there is a chance of very localised damaging wind gusts and perhaps even a tornado," Mr Wilkinson said in a statement.
"Tornado risks would be low as it's hard to tell when a tornado will occur precisely.
"But we do have the ingredients to form either a small tornado or a highly localised gust of damaging wind."
Electricity company Vector has warned customers to be prepared for possible power outages during storms today.
The company said repairs to any outages may be delayed during the wild weather.
In a statement this morning, Vector warned people to be wary of fallen power lines or damage to electrical equipment from fallen trees or branches.
"Vector will work hard to restore power as quickly as possible, however with storms the company has to prioritise multiple demands," the company said.
"The safety of the public and Vector's contracting field staff is of primary importance at all times."
The wet weather will move across other parts of the country tomorrow, including through the Waikato, Wairarapa, Wellington and Marlborough.
Wind gusts will cause headaches for Auckland commuters tomorrow morning, Mr Law said.
"Tomorrow morning it's still very windy for Auckland ... the low doesn't clear until the end of the day on Monday," he said.
"It gets better but later.
"It's a fairly blustery start to the day. Not great for commuting."
A road snowfall warning has also been issued for Lindis Pass and Milford Road.
Cold rain may turn to snow on the Lindis Pass above 700 metres this evening.
Two to four centimetres of snow is expected to build up near the summit this evening and early tomorrow.