UPDATED 9AM: The MetService has upgraded the Bay's rain watch to a warning as a low pressure system sitting west of New Zealand slowly approaches the country.

The system is expected to move across the country during today and tomorrow.

A rain warning is in place for the Bay of Plenty. Image: MetService.

During this time, periods of rain in a very humid northerly flow ahead of the low are expected across northern and western parts of the country.

In the Bay of Plenty, west of Whakatane and including Rotorua, a short period of intense heavy rain is expected early Tuesday morning.

"In the 6 hours from about 2am to 8am Tuesday expect 50 to 80mm to accumulate. Peak intensities 20 to 30mm per hour in possible thunderstorms," says a statement from the MetService.

"This amount of rain will cause streams and rivers to rise rapidly, slips and surface flooding, and could make driving conditions hazardous."

EARLIER:

A severe weather watch remains in place for the Bay of Plenty with predictions of heavy rain.

A low pressure system over the Tasman Sea is expected to slowly approach New Zealand during today and move across the country on Tuesday.

Heavy rain is expected in the Bay of Plenty today. Image: MetService.

Periods of rain in a very humid northerly flow ahead of the low are expected across northern and western parts of New Zealand.

According to the MetService's five day forecast, today's forecast is for rain developing, and a chance it good become heavy and thundery.

Tomorrow is looking slightly better with the MetService forecasting morning rain then the odd shower.

Things are then expected to brighten up from Wednesday.

'The heaviest rain is expected in Taranaki, Nelson, the Richmond Range, the Rai Valley and western Marlborough Sounds,” says a statement from the MetService.

For the Bay of Plenty, the watch in place is for the possibility that rainfall accumulations could approach warning thresholds.

'People are advised to keep up to date with forecasts in case any of these areas need to be included in the severe weather warning, or other areas are placed on watch.”