Weather forecast: Gales, thunderstorms, heavy rain and huge waves predicted

Batten down the hatches New Zealand - heavy rain, strong winds, thunderstorms and snow are on the way today, potentially followed by dangerous seas that could exceed 10 metres, forecasters say.

As well as strong wind and heavy rain, "squally thunderstorms and downpours are possible over the upper North Island including Auckland", MetService meteorologist Stephen Glassey warned.

"Snow is also likely to affect inland parts of Marlborough and North Island's Central Plateau for a time."

MetService is especially advising people from the Bay of Plenty to Northland to keep up to date with the latest severe weather information.

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Extensive severe weather watches have been issued, with potential for severe gales around Auckland, Northland and the Bay of Plenty from early morning Wednesday. The lower North Island and coastal Marlborough could also be hit by severe gales from this afternoon.

The extreme weather is thanks to an intense low that will track quickly across the North Island from the northwest.

Severe weather warnings haven't been issued yet - mainly because the system is moving so fast that rainfall won't stick around long enough to accumulate. There's also a bit of uncertainty about the low's exact path, according to MetService.

Severe weather watch areas for tomorrow. A deep low will move across the North Island. More details here https://t.co/bcFLKY4OUU. ^SG pic.twitter.com/lszHxtnZK5

— MetService (@MetService) June 4, 2019

The low moves away to the east of New Zealand early on Thursday. However, another bout of unsettled weather is expected when a front tracks northward over the country during Thursday and Friday, followed by strong, cold southwest winds and more snow in the mountains.

Forecaster Weatherwatch.co.nz is warning of huge waves on the west coast of the country from Thursday, and says fishing - from boats or rocks - is definitely a bad idea.

The forecaster predicts waves over Thursday, Friday and Saturday could exceed 10m out west.

Hi Auckland, we are following some weather tomorrow that @MetService are looking at. Forecasters are expecting a wet and windy Wednesday morning in Auckland with localised downpours and squally thunderstorms which are possible. https://t.co/OBotzhDBuX

— Auckland CDEM (@AucklandCDEM) June 4, 2019

The "dangerous seas" are thanks to the big southerly blast that will hit on Thursday and peak on Friday and Saturday, off the back of a large Tasman Sea low that will cross the North Island today, Weatherwatch said in its latest update.

With a large high over Australia and deep low pressure in the Southern Ocean, the pressure gradient between the two will produce big waves, Weatherwatch said.

Waves could exceed 10 metres on the west coast of the country later this week. Image / Weatherwatch.co.nz

"Those in Auckland are advised to avoid fishing on the rocks in the west, or being out on the water in the west, over Thursday, Friday and Saturday due to gale force marine conditions and these enormous seas.

"Meanwhile boaties and fisherman are advised by WeatherWatch.co.nz not to use dangerous river bars or harbours later this week like Greymouth or Manukau - or really any western marine entrances to the Tasman Sea - from Thursday to Saturday due to the dangers."

Those from Canterbury and Westland southwards should escape the bad weather, with a relatively settled, cold and frosty Wednesday, according to MetService. Frost could be severe inland.

Next week should start out more settled as a high pressure ridge moves on to New Zealand on Sunday, MetService said.