Auckland and other North Island areas may be blasted by heavy downpours on Monday afternoon.
Many areas are already experiencing heavy showers, with hail also possible in some places, MetService meteorologist Andy Best told Newshub.
The worst of the rain is expected down the spine of Northland through Auckland and down to the Waikato, Best said.
Downpours in some places may reach 40mm in an hour, the MetService warned.
The weather agency said that in the two hours leading up to 1:30pm, 13 lightning strikes were recorded in Auckland, while Waikato had 230.
The muggy, rainy conditions are a result of a low-pressure system in the Tasman Sea that is set to pass over the country on Tuesday.
The MetService currently has heavy rain warnings around Mt Taranaki, as well as in Nelson and northern Marlborough in the South Island.
Heavy rain watches are in place in the North Island in the Central High Country between Taranaki and Bay of Plenty, around the Bay of Plenty and the ranges of Gisborne.
In the South Island, there are heavy rain watches in the Canterbury High Country, the Canterbury Plains south of the Rakaia River, the ranges of Westland and in north Otago.
Warmer temperatures are also expected on Monday night, with "quite humid" conditions forecast for many in the north of the North Island, said Best.
More rain is also expected on Tuesday, before the humid conditions give way to cooler temperatures on Wednesday.
"Tuesday looks to be a stormy one for a lot of the country with heavy rain and strong winds affecting many areas," said Lewis Ferris, another MetService meteorologist.
"We quickly move into a cooler southwest flow and it's not long before the next rain band whips up western areas of the South Island on Thursday."
