The big drench: Long, slow system to bring heavy rain to NZ

Between 110 and 150mm of rain is expected to fall in the Nelson-Marlborough region from 9pm yesterday to 9pm tonight, MetService says. Photo / NZME

A lockdown low weather system, currently smothering western and northern areas of the South Island, will hang around the country for days, bringing up to 150mm of rain in certain cities.

The Nelson-Marlborough region has already been lashed with 70mm of rain in the last 12 hours and it's expected to hang around for another 12 hours.

MetService meteorologist Tom Adams said the calm, settled weather will come a halt for those in the North Island tomorrow as winter gets set to turn into spring.

"I guess slow moving is the key word," Adams said.

"Somewhat fortunately for much of the North Island, we've had a slow-moving ridge of high pressure.

"Unfortunately as that has slowly, slowly inched away from New Zealand we have a slow moving spell of bad weather, which is a low pressure system, which is sat out in the Tasman that really does take a while to cross the country.

"It's probably not going to cross our shores until Monday, Tuesday next week and not get completely clear of us until the latter half of next week or so."

The system had been hammering the western and northern areas of the South Island since yesterday but it given its slow nature, it won't hit the North Island until today.

"There could be rainfall rates of 15 to 25mm per hour. Having that amount of rain, both the accumulated amount over the 110, 150mm over 24 hours but also 25mm an hour have the potential to cause surface damage and slips."

About 70mm had fallen on Takaka Hill, while another 47 had fallen around the hills of Third House, in the last 12 hours.

The low system is entwined in a northerly flow so the temperatures will remain mild but is the reason why the upper South Island is in the gun.

Taranaki will next feel the brunt followed by the upper North Island, including Auckland, Coromandel and Bay of Plenty tomorrow before it settles around Hawke's Bay and Gisborne on Sunday.

"It's quite a broad feature, it's somewhat misleading to say it will be here as it will be affecting quite a wide region around it, but the centre of that front will be around the Cook Strait by midnight tonight and then moving across to western North Island through Saturday and eastern North Island on Sunday," Adams said.

"On Saturday it will probably be the wettest day for the upper North Island including Auckland and Coromandel, and also Bay of Plenty, the reason being those northerly winds shift further east, start getting into Hauraki Gulf and Bay of Plenty.

"The exclusions will be Taranaki which will be wet today and tomorrow."

As the system centres on Hawke's Bay and Gisborne on Sunday, it would still mean rain for surrounding areas, however the lower South Island was the only region to miss out.

There was also a risk of thunderstorms to certain areas, he said.

There would unlikely be any significant issue involving wind until around Monday or Tuesday as the system moves away from New Zealand.