On Monday, a shallow low is moving onto the North Island from the Tasman Sea, while a ridge of high pressure will be building over the South Island, says a MetService spokesperson.

"The low moves away to the east on Tuesday, and the ridge moves onto the North Island."

A front is expected to move northwards over much of the South Island during Wednesday, before stalling and weakening over northern Westland or Buller.

"This front is forecast to bring a period of rain to most of the South Island, and a period of strong or gale force northwesterlies ahead of the front," says a MetService spokesperson.

"There is moderate confidence rainfall amounts will reach warning criteria about Fiordland from Tuesday afternoon to Wednesday morning, and about Westland south of Otira, including the main divide of the Southern Alps, on Wednesday.

"Additionally, there is moderate confidence that northwesterlies will reach severe gale about exposed parts of inland Canterbury, inland Otago, Fiordland, Southland and Stewart Island on Tuesday and Wednesday morning, while there is low confidence for Wellington, Wairarapa and the Tararua District on Wednesday."

On Thursday, a low should approach central New Zealand from the Tasman Sea,while an associated trough crosses northern and central New Zealand.

"A period of rain is likely with the trough, which may briefly be heavy in northern and western regions.

"However, rainfall accumulations are not expected to approach warning criteria at this stage. In addition, a period of strong northerly winds are likely ahead of the trough."