Snow is forecast throughout the South Island on Wednesday night, with as much as 10 centimetres along some key routes.
MetService says the southerly front will move up the South Island, starting in the Milford area first about 2pm.
Up to 10cm of snow is predicted to fall on Porters Pass and with other falls expected on the Milford Road, Lindis Pass, Arthur's Pass and Lewis Pass.
Forecaster John Crouch says there have been thunderstorms in Southland and South Otago on Wednesday afternoon, but the coldest air is expected to move onto Southland in the evening.
The front is due in Canterbury on Wednesday evening and may make travel to or from the West Coast difficult.
In the North Island, MetService is forecasting up to 5cm of snow on the Desert Road early on Thursday and dustings on the Rimutaka Hill Road and the Napier-Taupo Road.