Severe thunderstorms could hit the North Island this evening, bringing heavy rain and strong winds.
It is the first of several bouts of wet weather forecast for the next week, and nearly all of the country is affected.
A severe thunderstorm watch is in place for Northland, Auckland, Great Barrier Island, Coromandel Peninsula, and Waikato from tonight.
"There is a subtropical low-pressure system sitting in the northern Tasman and that is going to sweep across the country later today for the upper North Island and then over much of the country tomorrow," MetService forecaster Ashlee Parkes said.
That could mean rainfall of 25 to 40mm per hour, which could cause surface flooding or flash flooding, especially around low-lying areas. Driving conditions could also be hazardous due to surface flooding and potential slips.
The rain band was expected to move over northern and central New Zealand tomorrow morning.
A heavy rain warning is in place for Coromandel from midnight, where up to 110mm of rain could fall.
Further heavy rain warnings have been issued for Gisborne - from Ruatoria northwards - on Monday morning and the Bay of Plenty.
Those are expected to be the worst-hit places, but thunderstorms could also move further south tomorrow afternoon.
More unsettled weather is forecast for later in the week.
Heavy rainfall is predicted for the west coast of the South Island early in the week, then another front from the Tasman Sea is expected to bring heavy rain and strong north-westerly winds for most of the country from late Tuesday through Wednesday.
"There are just waves of rain with heavy falls and strong winds for much of the week," Parkes said.
The only place to be spared from heavy rain would be the eastern side of the South Island.
After frosty conditions last week, the weather is expected to become warmer and muggier, and double-digits high are expected in most main centres overnight.
Napier and Gisborne could reach 21C during the day.