MetService has issued a severe weather outlook for New Zealand.

"An unsettled northwest flow covers the country on Tuesday, bringing rain or showers to western regions," says a MetService spokesperson. 

"There is low confidence rainfall amounts will reach warning criteria in Buller and Westland, mainly about the ranges."

MetService says cold and showery southwesterlies will spread northwards over the country during Wednesday, then ease on Thursday as a ridge of high pressure brings more settled weather.

"Note, although snowfall amounts are not expected to approach warning criteria, some high roads may be affected by snow for a time this week."

The flow is forecast to turn northwest on Friday as the ridge moves to the east of New Zealand and a frontal system approaches from the Tasman Sea.

Heavy snow for Fiordland

A cold and unsettled west to northwest will be affecting western parts of the South Island through to Tuesday.

"A Heavy Snow Watch is now in place for Fiordland during Monday."

The Heavy Snow Watch for Fiordland is valid from 1am  - 3pm Monday August 14.  Rain, with possible thunderstorms, is expected to turn to snow above 300 metres. Heavy snowfall is possible above 500 metres, where amounts may approach warning criteria.