Heavy rain is being forecast for Northland, Coromandel Peninsula, Bay of Plenty and Gisborne from Tolaga Bay northwards, warns the MetService.

A complex low pressure system over the northern Tasman Sea was forecast to move east across the upper North Island last night and through Christmas Day.

"Fronts associated with this low are spreading south across the North Island bringing periods of heavy rain to many places and thunderstorms are also possible.

"The heaviest rain is expected in Northland, Coromandel Peninsula, Bay of Plenty and Gisborne from Tolaga Bay northwards and a Heavy Rain Warning is in force," says the MetService.

"This is likely to be a significant event for these areas, especially for parts of Coromandel Peninsula and western Bay of Plenty, where further flooding is likely."

Note, the combination of the approaching low, strong onshore winds and King Tides late Monday and during Christmas Day could cause some coastal flooding about eastern coasts.

Heavy Rain Warning

Heavy rain may cause streams and rivers to rise rapidly. Surface flooding and slips are also possible and driving conditions may be hazardous.

Area: Coromandel Peninsula
Valid: 12 hours from 8pm Monday to 8am Tuesday
Forecast: Periods of heavy rain. A further 60 to 90mm expected to accumulate on top of what has already fallen. Peak intensities of 15 to 25mm per hour or possibly higher. Thunderstorms also possible.

Please note that showers are expected to continue behind the main rain band during Christmas Day with isolated thunderstorms possible.

Area: Bay Of Plenty, including Rotorua, also Gisborne from Tolaga Bay northwards
Valid: 14 hours from 8pm Monday to 10am Tuesday
Forecast: Periods of rain, heavy at times. Expect 80 to 100mm to accumulate on top of what has already fallen, especially in areas west of about Whakatane. Peak intensities 15 to 25mm per hour.

Please note that showers are expected to continue behind the main rain band during Christmas Day with isolated thunderstorms possible.