A cold southwesterly flow will cover New Zealand on Tuesday and Wednesday, says a MetService spokesperson.

The cold air flow will have an embedded front moving northwards over the South Island late on Tuesday and Wednesday, and over the North Island early on Thursday.

"This front is expected to bring a period of rain to the South Island later Tuesday and Wednesday."

"Snow [will be] lowering to low levels in the south and east of the South Island on Wednesday, and possibly the southern and central North Island on Thursday morning.

"This snow is likely to affect high country roads, and there is low confidence that snowfall amounts may reach warning criteria above 500 metres in parts of Central Otago, inland Dunedin and Fiordland on Wednesday morning."

MetService says a ridge of high pressure then spreads over New Zealand later Thursday and Friday, but a new front may move onto the South Island late Friday.