A heavy rain warning is in place for Gisborne/Tairawhiti and Westland.

"Tonight and Monday, a humid east to northeast flow is expected to affect Gisborne/Tairawhiti, bringing a period of heavy rain to the region where a Warning and Watch are in force," says a MetService spokesperson.

"An active trough is forecast to move across the Westland and Grey Districts, bringing a period of heavy rain with localised thunderstorms and downpours possible. A Watch is now in force."

The heavy rain watch for Gisborne/Tairawhiti south of Tolaga Bay is valid for 13 hours from 3am - 4pm on Monday January 22. A period of heavy rain is forecast, and amounts may approach warning criteria.

The heavy rain watch for Westland and Grey Districts is valid for 13 hours from 11pm Sunday January 21 to 12pm Monday January 22. Periods of heavy rain and amounts may approach warning criteria and thunderstorms and localised downpours of 25 to 35 mm per hour are possible.

Tuesday January 23
On Tuesday, a sub-tropical low is forecast to move southeastwards away from the North Island, with low confidence of a period of severe northeast gales about the Chatham Islands.

Meanwhile, a trough over southern New Zealand moves northeastwards over the remainder of the country. Strong or gale southwest winds are likely in many places following the trough, along with cooler, showery weather.

There is low confidence that rainfall amounts will reach warning criteria about the Grey and Westland Districts from the glaciers northwards, and Buller.

In addition, there is low confidence that southwest winds will rise to severe gale for a time about Stewart Island and coastal parts of Clutha and Dunedin.

Wednesday January 24
The strong, cool southwest flow over the country eases as a ridge of high pressure builds over the South Island.

Thursday January 25
MetService says a ridge of high pressure is forecast to remain over much of the country on Thursday.

Friday January 26
The ridge of high pressure is forecast to retreat to the North Island, allowing a front preceded by a strong northerly flow to approach the South Island.

For Fiordland, there is low confidence of warnable amounts of rain, and moderate confidence northerly winds will rise to severe gale.