Today, a slow-moving low west of northern New Zealand would spread a humid northerly flow, bringing periods of heavy rain in the upper North Island, MetService said.
There was also a broad low thunderstorm risk over the northern half of the island.
But for Auckland, Coromandel Peninsula and Waikato there was a moderate risk before sunrise.
"Then during the morning the thunderstorms risk in Northland increases to moderate, bringing localised heavy rain with intensities of 10 to 25mm per hour.
"There is also a risk some of these thunderstorms could become severe, bringing localised downpours of 25 to 40mm/hr."
A heavy rain watch was also in place for Auckland and Great Barrier Island until 7am Tuesday.
A similar watch was also in place for Waikato, Bay of Plenty and Gisborne from Tolaga Bay northward, MetService said.
Forecasts for December 25 also looked grim in the North Island.
On Christmas Day, a complex low-pressure system was forecast to move east across the North Island, bringing periods of heavy rain to many places.
"There is high confidence of rain accumulations reaching warning criteria in Northland, Auckland, Coromandel Peninsula, Waikato, Bay of Plenty and northern Gisborne, and moderate confidence in the remaining areas from Taranaki across to northern Hawke's Bay northwards during Christmas day as indicated on the chart."
The low was expected to move away on Boxing Day and a ridge would build over the country, remaining slow moving through to Friday bringing settled weather.
Your weather
Whangārei: Rain, heavy at times with possible thunderstorms. Northeasterlies.
High 23C Low 17C
Auckland: Rain, heavy at times with possible thunderstorms. Northeasterlies, becoming strong evening.
High 22C Low 18C
Tauranga: Rain, heavy at times. Easterlies.
High 22C Low 19C
Whanganui: Cloudy with a few spits. Occasional rain from afternoon. Gusty easterlies.
High 26C Low 16C
Napier: Periods of rain, chance heavy later. Northeast winds.
High 21C Low 18C
Christchurch: Mainly fine, morning and evening cloud. Northeasterlies, strong for a time afternoon.
High 20C Low 14C
Dunedin: Fine with northeasterlies.
High 21C Low 13C
Source: MetService
