A thick layer of new snow has prompted dangerous avalanche warnings in two South Island alpine ranges.

The Avalanche Advisory has issued alerts for a considerable risk for alpine terrain at Two Thumb Range and for high alpine terrain at Aoraki / Mount Cook.

A large storm slab avalanche was observed on the Richmond Range in the last few days with multiple loose, wet avalanches sighted from the valley floor in the southern end of the Two Thumb range.

A moderate avalanche risk alert has been issued for Tongariro, Arthur's Pass, Craigieburn Range, Mt Hutt, Ohau, Queenstown, Wānaka, Fiordland, and Mount Aspiring National Park.

Motorists are also being urged to drive with caution because of snow.

In the South Island, snow is forecast down to 700 metres on the Milford Road, and on the Lewis and Porters passes.

A brief fall is predicted as low as 800m on the Lindis Pass.

This afternoon snow will lower to 800m on the Crown Range Road and to 500m tonight.

On the Desert Road in the North Island, snow is expected to 700m early tomorrow morning.

Up to eight centimetres of snow is predicted to settle on the road.

Meanwhile, strong wind watches are in place for the top and bottom of the country - starting from Wednesday.

In the North Island, MetService has issued strong wind watches for Northland, Auckland, Coromandel, Waikato, north of Huntly, and coastal areas from Tokomaru Bay to Wairoa.

Down south, a watch was in place for Dunedin and North Otago, Clutha and Southland, as well as Stewart Island.

"A strong southwest flow is expected to develop over the country during Wednesday and Thursday," MetService said.

"People are advised to stay up to date with the latest forecasts in case any changes are made."