Rain and snow are being forecast for New Zealand this week.

MetService are forecasting a strong moist northwesterly flow bringing significant amounts of rain to many areas across New Zealand from Wednesday to Friday.

"The flow turns more westerly on Saturday," says a MetService spokesperson.

"Conditions are likely to cool significantly over the South Island with snow expected in many mountain areas, especially on Friday.

"Note that this is likely to be the first heavy snow of the season in these areas and snow may lower to levels where it can cause stress to livestock."

On Wednesday, rainfall accumulations may reach warning amounts for Westland and Buller, and on Thursday rainfall accumulations may reach warning amounts for Westland, Buller and the Tararua Ranges.

"On Friday, there is a moderate confidence of rainfall accumulations reaching warning amounts for Westland, Buller and the Tararua Ranges and a low confidence of warning amounts of rain for Mount Taranaki, the west of the central North Island high country and the headwaters of the Canterbury lakes and rivers.

"There is a moderate confidence of snow accumulations reaching warning amounts above
about 800 metres for the Canterbury high country south of the Rangitata River and for Otago west of about Alexandra.

"Note that this is likely to be the first heavy snow of the season in these areas and it may cause stress to livestock."

Severe northwesterly gales are also expected on Friday about the Marlborough Sounds, Wellington and Wairarapa including the Tararua District.

"On Saturday there is a moderate confidence of rainfall accumulations reaching warning amounts for Westland, Buller and the Tararua Ranges and a low risk of rainfall accumulations reaching warning amounts for Mount Taranaki, the west of the central North Island high country and the headwaters of the Canterbury lakes and rivers.

"There is a moderate confidence of severe northwesterly gales about the Marlborough Sounds, Wellington and Wairarapa including the Tararua District."