UPDATED 9.02AM: The MetService has upgraded the Bay's weather watch to a severe weather warning.

A warm moist northerly flow over New Zealand is expected to strengthen today as a front over the Tasman Sea approaches the country.

A severe weather warning has been issued for the Bay.

The front is expected to move onto the South Island tonight and onto the North Island Wednesday, bringing rain to many parts of the country.

"From Tuesday evening to Wednesday morning, heavy rain is expected in the ranges of northwest Nelson and Buller where 90mm of rain may accumulate, while the ranges of Westland south of Otira may accumulate 150m," says the MetService.

"Rain is expected to become heavy about the ranges of Bay of Plenty east of Whakatane from around midday Wednesday to Thursday morning and 180mm of rain may accumulate."

Rain is expected to become heavy from around midday Wednesday and continue through to Thursday morning.

In the 24 hours from midday Wednesday to midday Thursday, 150mm to 180mm of rain is expected to accumulate. Peak intensities of 15mm to 25mm per hour are possible.

People are advised that this amount of rain will cause rivers and streams to rise rapidly, with slips and surface flooding possible.

Heavy Rain Warning issued for BOP, NLSN, BULR, WSTLD https://t.co/B6xFhW3VGB

— Severe Weather Info (@MetServiceWARN) June 20, 2016

Earlier:

The MetService has issued a severe weather watch for the Bay of Plenty with heavy rain expected tomorrow.

'A warm moist northerly flow over New Zealand strengthens on today as a front over the Tasman Sea approaches the country,” says a statement from the MetService.

Looks like rain is set to hit the region for the rest of the week. Image: MetService.

'The front is expected to move onto the South Island overnight Tuesday, then across the country during Wednesday.”

This front should bring periods of heavy rain to the west and north of the country on Wednesday, followed by an unsettled northwest flow.

The watch is for the possibility of rainfall accumulations reaching warning criteria in the following areas.

In the Bay of Plenty, from about midday Wednesday to overnight (Wednesday), is when the heaviest falls are expected.

'People are advised to stay up to date with the latest forecasts in case parts of this Watch are upgraded to a Warning or further areas are added.”

The watch will be reviewed by 10am.

Keep checking SunLive for any updates.