Small tornadoes are also possible, MetService warned.
WeatherWatch.co.nz have alerted heavy downpours with squally winds and possible thunderstorms are expected between now and 4.15pm in Taupo, Rotorua and Tokoroa.
The thunderstorms are being caused by an unstable northwest flow sitting over central and northern New Zealand.
MetService meteorologist Peter Little urged people to take care.
"There could be more tornadoes, there could be more wind gusts come out of these storms. There could be large hail in places, so people if they are out and about just need to be aware. Thunderstorms can be dangerous, so if they see lightning or hear thunder and it is approaching then they should take shelter indoors."
Gisborne, Hawkes Bay, Wairarapa, southern parts of Wellington and inland parts of Buller and Nelson are expected to miss the wild weather impacting the rest of the North Island.
Meanwhile, the country should rug up as the temperature is expected to drop over the next couple of nights as a big storm brings Antarctic winds to the country.
WeatherWatch said the storm was working with other lows to dredge up an Antarctic southerly tomorrow.
That will bring some snow to low levels in the south, and a brief nationwide temperature drop.
Head analyst Philip Duncan was quick to emphasise that the storm was "well south'' of New Zealand.
Last week, there was some confusion among members of the public about which weather systems were affecting different parts of the country and when.
"The cold front arrives [tomorrow] morning, but the coldest air comes in [tomorrow] night and sea level snow is possible in Milford Sound; while a few flurries will make it in to Southland and Otago - possibly down to 100m.''
Duncan said the good news was that the showers would not be heavy, or accumulate as much.
Those in Dunedin may get a few flurries tomorrow night and through to Wednesday morning at the peak of the cold snap.
"One big positive is that the enormous belt of low pressure is moving eastwards and a large high from Aussie will encourage this cold snap to not only move through fast, but it will also push the very coldest of the air to just slightly south of the South Island.''
Most areas can expect to see temperature highs drop to single digits tomorrow and Wednesday, before warmer air comes in from Australia by Wednesday night and nationwide by Thursday, he said.
"By the end of Wednesday, this enormous area of low pressure ... will finally ease after the colder southerly portion blows through.''
The MetService issued a thunderstorm outlook just after 10am today and is in place until midnight.
The forecast comes after huge weather action over the last 24 hours; including heavy rains and strong winds lashing Auckland and the Waikato and a tornado ripping through New Plymouth this morning.
Roofs were uplifted and trees ripped out of the ground when the weather suddenly took a turn from about 8pm last night.
Lee said the slightly warmer-than-average temperatures have also contributed to the thunderstorm activity.
NZ Fire and Emergency services were called out to 20 incidents around Auckland and Waikato. Five houses in St Heliers were badly damaged and a preschool had part of its roof ripped off.
The Cloud on Queen's Wharf on the Auckland waterfront has also been left in tatters; with a large chunk of it flying off during the weather chaos. Shipping containers down the road also looked more like Lego blocks as they were left toppled over in the aftermath of the storm.
Although a house in Kumeu was struck by lightning, no injuries were reported, say authorities.
And, earlier this morning, emergency crews were called to New Plymouth after several reports of a tornado.
A vehicle was struck by a trampoline on State Highway 3 and a woman was injured when the tornado blew through their Taranaki house.
THUNDERSTORM ACTIVITY CONTINUES
There have been a lot of thunderstorms brewing over the Tasman Sea during the past few...
Posted by MetService New Zealand on Sunday, 11 August 2019
