MetService is forecasting a northeast flow will be covering New Zealand on Tuesday as a ridge of high pressure over the North Island continues to move slowly eastwards.

"The flow turns northwesterly over the South Island during Wednesday as a trough crosses the area," says a MetService spokesperson.

"A front is forecast to move onto the south of the South Island from the southwest on Thursday, then onto the North Island on Friday while a deepening low approaches the North Island from the Tasman Sea. A change to colder southwesterlies follows the front."

The MetService says the trough and front are expected to bring periods of rain or showers to the South Island from Monday to Friday, and for the North Island on Thursday and Friday.

"Snow is also possible to 500 metres in the south and east of the South Island late Thursday and Friday," says a MetService spokesperson.

"There is moderate confidence of rainfall accumulations reaching warning criteria in central and northern Westland, Buller and northwest Tasman on Wednesday and Thursday, with low confidence for southern Westland and Fiordland.

"For the North Island, there is low confidence a heavy rain warning will be required on Friday for central parts of the North Island as indicated on the chart, and also about the Tararua Range."