MetService says a new area of low pressure over the Tasman Sea is expected to move eastwards over the North Island during Tuesday, followed by a strong southerly flow.

Meanwhile, a ridge of high pressure over southern New Zealand spreads onto central New Zealand on Tuesday and northern New Zealand on Thursday, with southerly winds easing.

Also on Thursday, a front approaches southern New Zealand from the Tasman Sea. This front should move onto Fiordland on Friday, and is preceded by heavy rain in the west, and northwest gales in exposed parts of southern New Zealand.

On Tuesday, there is moderate confidence of warnable amounts of southerly rain for Gisborne and the Wairoa District of Hawke's Bay.

On Thursday, there is low confidence a heavy rain warning will be needed for Fiordland.

On Friday, the confidence of rainfall amounts reaching warning criteria in Fiordland increases to moderate, and there's also moderate confidence of heavy rain for Westland from the glaciers southwards, especially about the ranges.

A Heavy Rain Watch is in place for these two regions:

Area: Bay of Plenty from Rotorua northwards
Valid: 16 hours from 2:00pm Mon 22 May to 6:00am Tue 23 May
Forecast: Periods of heavy rain with thunderstorms possible. Rainfall amounts may approach warning criteria, mainly about the eastern ranges where there could be downpours possible.

Area: Gisborne
Valid: 24 hours from 12:00am Tue 23 May to 12:00am Wed 24 May
Forecast: Periods of heavy rain. Rainfall amounts may approach warning criteria.
Note, heavy rain is expected to develop in the north overnight Monday and then spread farther south during Tuesday morning.