MetService has issued a heavy rain watch for the eastern Bay of Plenty and Tasman regions.

A low pressure system in the Tasman Sea is expected to move eastwards across New Zealand during Wednesday, bringing rain to central and northern regions.

"The heaviest rain is expected over the Tasman District and eastern Bay of Plenty, where Heavy Rain Watches are now in force," says a MetService spokesperson.

A Heavy Rain Watch in place in the eastern ranges of Bay of Plenty is valid for 13 hours from 9am - 10pm Wednesday May 15. A period of heavy rain with possible thunderstorms, and amounts may approach warning criteria.

A Heavy Rain Watch in place for the Tasman area and west of Motueka is valid for 13 hours from 4am - 5pm on Wednesday. A period of heavy rain, and amounts may approach warning criteria.

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

On Thursday a low over the North Island should move off to the east and is followed by a southwest flow. Later in the day, winds are expected to turn northwest and strengthen about the southern South Island.

Heavy rain may continue to fall in eastern Bay of Plenty during Thursday morning and there is low confidence that a rain warning will extend into Thursday.

For southern Fiordland and Stewart Island, there is low confidence that the northwest winds will increase to warning strength by the end of the day.

A front, moving northeast, is expected to move onto the South Island during Friday, bringing rain to the west and strong west to northwest winds to the south and east. A ridge of high pressure should lie over the North Island.

There is low confidence that heavy rain in Fiordland will require a warning on Friday. For Fiordland and Stewart Island there is low confidence that a wind warning will be needed for severe gale west to northwest winds. About inland areas of mainland Southland, Otago and Canterbury there is also low confidence that northwest winds will reach warning strength.

On Saturday, May 18, the front should continue moving northeast over the South Island and weaken, reaching the southern North Island late in the day.

There is a chance a low will develop on the front east of the South Island. There is likely to be rain for much of the South Island and southern North Island. Strong northwest winds ahead of the front should ease in the afternoon.

Snow is possible about the southern and central South Island, but the level it falls to is uncertain at this stage.

There is moderate confidence that a rainfall warning will be needed for Fiordland and the ranges of the Westland District on Saturday. Severe gale northwesterlies may affect inland areas Canterbury, southern Marlborough, the Marlborough Sounds, Wellington and southern Wairarapa with low confidence that a warning will be needed.

here is low confidence that snowfalls above 700 metres in Otago and South Canterbury will be heavy enough to require a warning.

During Sunday, a weak ridge of high pressure is expected to form over New Zealand. Rain may continue about the west of the South Island. There is low confidence that a heavy rainfall warning will be needed for the ranges of Westland District.