Thundery downpours are being forecast for the region as the MetService upgrades its severe weather watch to a warning.

A low and associated fronts over the Tasman Sea are moving southeastwards, and will bring rain to much of the North Island.

The heaviest falls are likely to be in Waikato, Coromandel Peninsula and Bay of Plenty, and a Heavy Rain Warning is now in place for these areas.

'Up to 150mm of rain could accumulate in a relatively brief period, with localised thundery downpours,” says a statement from the weather organisation.

'Please note that a watch is in force for significant heavy rain for Northland, Auckland, Waitomo, Taranaki, Taumarunui, Taupo, Gisborne and northern Hawke's Bay.

'The initial burst of rain should ease during Wednesday, but further rain is expected during Thursday and Friday.”

Rain is forecast to become heavy in the Bay of Plenty tonight.

In the 18 hours from 9pm tonight until 3pm Wednesday, people are being warned to expect 100 to 130mm of rain to accumulate, possibly even 150mm in the west.

Peak intensities of 25 to 40mm/hr overnight and Wednesday morning, says the MetService.