Severe weather warnings are being issued as a front over the South Island on Sunday moves north, with MetService saying the humid conditions are "favourable for thunderstorm development".
The front came off the Tasman Sea on Sunday morning, bringing heavy northerly rain to the West Coast.
A heavy rain warning has been issued until 11pm on Sunday for the ranges of Nelson west of Motueka, along with the ranges of Buller and Westland from Otira northwards.
"Expect 70 to 100 mm of rain. Peak rates of 10 to 15mm/h," MetService says.
Another warning has been issued for Richmond Range and Rai Valley where 75 to 100mm of rain is expected, with peaks of 10 to 20mm/h from this evening.
This slow-moving front is then expected to move over the west of the North Island on Monday.
MetService warns from early tomorrow conditions in the west of North Island are favourable for thunderstorm development, with Taranaki and Waitomo especially at risk.
"This front has the potential to bring localised downpours to Taranaki and coastal Waitomo on Monday morning," MetService warns.
"Rainfall rates of 25 to 40 mm/h are possible. Rainfall of this intensity can cause surface and/or flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, and may also lead to slips. Driving conditions will also be hazardous with surface flooding and poor visibility in heavy rain."