A heavy rain watch issued for the Bay of Plenty is now being upgraded to a warning.

The MetService issued a severe weather watch for heavy rain on Tuesday.

In the latest update today, the MetService says heavy rain is forecast for parts of central and northern New Zealand.

"A slow moving low north of New Zealand directs an unsettled and humid east to northeast flow across the country," says a spokesperson for the weather organisation.

"Fronts embedded in this flow are forecast to move over the upper North Island during Wednesday and Thursday, delivering rain or showers to many places, with some regions receiving heavy rain.

"A Heavy Rain Warning remains in force for the Coromandel Peninsula and a Heavy Rain Warning is now in place for the western Bay of Plenty.

"Watches remain in force for Auckland, central Bay of Plenty, northern Gisborne and now includes northwest Tasman.

"People are advised to keep up to date with the latest forecasts in case any Watches are upgraded to Warnings, or further areas are added."

Heavy Rain Warning - Orange
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 and western Bay of Plenty about and west of Tauranga including Kaimai-Mamaku ranges
Valid: 24 hours from 9am Wed 14 Dec to 9am Thu 15 Dec
Forecast: Expect 140 to 180 mm of rain about the ranges, with 70 to 100 mm near the coast. Peak rates of 15 to 20 mm/h expected about the ranges.

Heavy Rain Watch
Area: Auckland
Valid: 17 hours from 10am Wed 14 Dec to 3am Thu 15 Dec
Forecast: Periods of heavy rain. Rainfall amounts may approach warning criteria, especially about the Hunua ranges.

Area: Bay of Plenty between Tauranga and Whakatane including Rotorua
Valid: 30 hours from 1pm Wed 14 Dec to 7m Thu 15 Dec
Forecast: Periods of heavy rain. Rainfall amounts may approach warning criteria, mainly inland about the ranges.

Area: Gisborne ranges north of Tolaga Bay
Valid: 12 hours from 3am Thu 15 Dec to 3pm Thu 15 Dec
Forecast: First period of heavy rain today but rainfall amounts are unlikely approach warning criteria. Second period of heavy rain during Thursday where rainfall amounts may approach warning criteria.

Area: Tasman northwest of Motueka
Valid: 18 hours from 12am Thu 15 Dec to 6pm Thu 15 Dec
Forecast: Periods of heavy rain. Rainfall amounts may approach warning criteria.