A severe weather watch is in force the Bay of Plenty and Rotorua which could mean heavy rain at times and possible isolated thunderstorms are on the way.

The MetService said the watch also includes the Waikato, Coromandel Peninsula and northern Gisborne, Northland and Auckland, including Great Barrier Island.

In a written statement, the MetService said a complex low-pressure system and associated fronts were forecast to slowly spread onto the upper North Island from today through to Christmas Day, then move away to the east on Boxing Day.

"The system is expected to bring periods of heavy rain and possible thunderstorms to northern parts of the North Island, with the possibility of short periods of intense rain.

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"Total rainfalls during this extended period could exceed 100mm in parts of eastern Northland, northern Auckland, Coromandel Peninsula, Bay of Plenty and northern Gisborne, especially about the hills and ranges, including near the Kaimai Range."

The MetService said there was still some uncertainty about the timing and passage of this low and associated fronts which would affect the distribution of the heaviest rainfall.

"Therefore, people in these areas are advised to stay up to date with the latest forecasts in case parts for this watch are upgraded to a full warning or new areas are added.

"Note, the combination of the approaching low, strong onshore winds and King Tides late Monday and during Christmas Day could cause some coastal inundation about eastern coasts, including parts of Auckland such as Tamaki Drive and about the Firth of Thames."