Heavy rain warnings have been issued over the top half of the North Island, and motorists are warned to watch for possible surface flooding.
There is also a possibility of hail in some areas today.
The MetService has severe weather warnings in place for Northland and Auckland, where heavy rain is expected between Kawakawa and Orewa.
Up to 75mm to 90mm of rain may fall, especially in the east, from 8am to 11pm.
The warning also includes Great Barrier Island.
MetService meteorologist Sarah Haddon said: "There's a low pressure system sitting just to the northwest of New Zealand at the moment.
"Then it's just going to slowly roll east and as it does that, it's kind of bringing an easterly flow onto the top half of the North Island.
"Embedded in that is periods of heavy rain and thundery squalls as well."
Rain is already falling heavily in parts of Auckland and the NZ Transport Agency warned drivers to take care
Meanwhile, rugby fans travelling to the first Lions match tomorrow, in Whangarei, are encouraged to drive to the conditions.
Haddon said most of the rain should be gone from the area by tomorrow.'
Those heading out for the Queen's Birthday long weekend should keep an eye on the weather, however, as some places are expected to be cloudy.
"Your usual places that are delightful and sunny - like the East Coast of the North Island, Gisborne, Hawke's Bay - they're going to be relatively cloudy with showers all of today, all of tomorrow and kind of into Sunday."