UPDATED 4.55PM: Motorists around the country are being warned to take care as wild weather continues.

A severe thunderstorm watch has been issued for Nelson, Buller and Westland with a heavy snow forecast for Southland, Otago, and the alpine passes.

The Transport Agency is warning motorists of wild, windy, conditions with debris blown onto many roads, especially on the West Coast and in Waikato, where police say unnecessary travel should be avoided.

Stormy conditions closed low lying roads north of Wellington at high tide.

Police said part of State Highway 58 through Whitby has reopened, as well as Grays Road at Plimmerton, but a high tide at midnight could see them close again.

Drivers are being told to carry chains on the South Island's alpine passes as well as on the Lindis Pass as sleet and snow continue.

Chains are essential on Arthur's Pass, with towing of vehicles now prohibited.

There's surface flooding on State Highway 65 south of Murchison.