An active trough of low pressure is moving eastwards across the country, bringing heavy rain to the lower North Island and strong winds to the east of that island.
Heavy snow is possible for Canterbury and Marlborough on Sunday, and then for the southern South Island on Monday, affecting most alpine roads and other main highways.
The cold snap would first be felt in Canterbury, where snow was forecast for inland parts of the region, north of Waipara and Marlborough, south of State Highway 63 from Sunday morning until 10pm, MetService said.
Rain turning to snow was set to affect alpine highways Porters, Arthur's, Lewis and Lindis throughout the day.
Monday sees a rush of cold air down south, bringing snow to Southland and Otago throughout the day. The snow is expected to be heavier above 400 metres, but a few flakes could make it to sea level, the forecaster says.
A heavy snow watch remained in place for Central Otago south of Lake Wakatipu and Roxburgh, Clutha, Southland and Fiordland, south of George Sound from 2am until 5pm.
There would be frequent showers and strong southwesterlies, with snow to low levels.
"Expect heavy snow at times above 400 metres, where snowfall may approach warning criteria in places," MetService said.
Snow showers were also forecast for the Crown Range Road, linking Queenstown and Wānaka, the Milford Road (SH94) in Southland, and the Lindis Pass (SH8) in Central Otago throughout Monday.
Snow showers were set to affect Dunedin's Northern Motorway (State Highway 1, Dunedin to Waitati) for most of Monday from 10am until midnight.
State Highways affected
South Island
Arthurs Pass (SH73)
Milford Road (SH94)
North Island
Remutaka Hill Road (SH2)
From 1am until 6am on Monday
Some snow flurries may affect the summit overnight but little if any snow is expected to settle on the road.
Desert Road (SH1)
From 7am until 10pm on Monday
Snow showers are expected above about 1000 metres throughout the day where 1cm to 2cm of snow may accumulate at times.
