North Island set for severe thunderstorms, flash floods: MetService

Power is out in parts of the upper North Island on Tuesday night as bad weather passes over.

Energy company Vector said it is investigating the cause of the outage.

Much of the upper North Island is under a severe thunderstorm watch until midnight.

In Bay of Plenty, 141 in Oropi are without power. An outage in Pukehina that earlier affected more than 200 is being resolved and now less than 30 remain without power.

In Northland, an outage is affecting part of Pataua and Parua Bay near Whangarei Heads.

An outage in Auckland's Papatoetoe suburb has been resolved, but trees on a powerline in Waitakere has resulted in an outage near Bethells Road.

Northland, Auckland and Great Barrier Island, Coromandel Peninsula, Waikato, Waitomo and Taranaki are forecast to be hit by severe thunderstorms and possible surface flooding late on Tuesday.

MetService predicts that the thunderstorms could bring downpours of between 25 and 40mm per hour.

It said surface flooding and flash flooding were possible.

Niwa principal scientist Chris Brandolino told Checkpoint the place you don't want to be when there is lightning in and around your area is outside.

"You definitely don't want to be on the water or on the beach or under a single tree in a paddock. Lightning is lazy, lightning is going to go for the tallest thing out there."

He said the safest place to be is inside, away from windows: "There is an expression - when thunder roars, head indoors."

Auckland City had been blanketed in a thick fog for much of Tuesday morning. Photo/RNZ

Brandolino explained that thunder is a result of lightning.

"Lightning is so hot - roughly five times hotter than the surface of the sun - it expands the air quite violently and then the air comes back, and it's that process that creates the thunder and loud noise."

Severe thunderstorm watches are in place for Northland, Auckland, Great Barrier Island and western Waikato.

Northern Coromandel Peninsula, Waitomo and Taranaki have also been included.

A heavy rain watch has also been issued for the Bay of Plenty.

The thunderstorm watches are in place from 4pm until midnight, with Bay of Plenty's heavy rain watch in place from 7pm until 6am Wednesday.

MetService said conditions should begin to ease in western areas from 9pm.

This story was first published in RNZ.