MetService warns "all the ingredients for thunderstorms are in place" over the North Island today as it monitors areas at risk of downpours.
Multiple places have been put on severe thunderstorm watch until Wednesday, with heavy rain warnings also in place for the Far North and Taranaki.
"Moist unstable northerlies are expected to bring frequent showers and some thunderstorms to Northland and the far north of Auckland this afternoon and evening," MetService warns.
"Some of these may be SEVERE bringing downpours with rainfall amounts of 25 to 40mm per hour."
The North Island is already seeing the impact, with 36mm of rain recorded at the Hamilton Airport between 5-7am.
And a lightning strike map shows thunderstorms are rumbling over Waikato - including a large number of cloud-to-ground strikes.
MetService says there's likely to be more rain before the week is up.
"There is a low pressure system which is forecast to roll across the country through Wednesday and Thursday, bringing plenty of rain to central and northern NZ," it says on Facebook.
"Current information has the low pulling away on Friday, but there is a very fast-moving front approaching from the southwest which zooms up the country.
"This brings a burst of gusty southwesterly winds and some showers but gets rid of the high humidity air."