Heavy rain is being forecast for the west and north of the South Island and parts of the North Island until Saturday morning.

The Metservice has issued a severe weather watch for heavy rain for the Bay of Plenty, including Rotorua.

"A series of fronts embedded in a humid north to northwest flow is bringing periods of heavy rain to parts of New Zealand today and early Saturday. Note, for the central North Island areas, downpours are also possible," says the latest update from the weather organisation.

"Warnings and Watches for heavy rain are in place.

"Please keep up to date with the latest forecasts as changes may be made."

Heavy Rain Watch
Area: Waikato, Waitomo, Taumarunui, Taupo, Tongariro National Park and the Kaimanawa Mountains
Valid: 22 hours from 9am Fri 29 Dec to 7am Sat 30 Dec
Forecast: Periods of heavy rain, with thunderstorms possible, and amounts may approach broadscale warning criteria or even exceed warning amounts in localised areas.
Note: downpours of 25 to 40 mm/h are possible with or without thunderstorms from this afternoon.

Area: Bay of Plenty including Rotorua
Valid: 25 hours from 9am Fri 29 Dec to 10am Sat 30 Dec
Forecast: Periods of heavy rain, with thunderstorms possible, and amounts may approach broadscale warning criteria or even exceed warning amounts in localised areas.
Note: downpours of 25 to 40 mm/h are possible with or without thunderstorms from this afternoon.

Area: The Tararua Range
Valid: 18 hours from 9am Fri 29 Dec to 3am Sat 30 Dec
Forecast: Periods of heavy rain, and amounts may approach warning criteria.

Area: Richmond and Bryant ranges, including the Rai Valley
Valid: 11 hours from 9am Fri 29 Dec to 8pm Fri 29 Dec
Forecast: Periods of heavy rain, and amounts may approach warning criteria.

Area: Buller
Valid: 22 hours from 9am Fri 29 Dec to 7am Sat 30 Dec
Forecast: Periods of heavy rain, with thunderstorms possible. Rainfall amounts may approach warning criteria. The rain should ease for a time tonight.

Area: Fiordland, north of Doubtful Sound
Valid: 20 hours from 9am Fri 29 Dec to 5am Sat 30 Dec
Forecast: Periods of heavy rain, with thunderstorms possible. Rainfall amounts may approach warning criteria.
Note: another burst of heavy rain is expected Saturday night and early Sunday morning.