Snow to low levels, damaging gales, thunderstorms and monster swells are on their way.

They're expected to batter the country over the next 48 hours.

Much of the South Island and the bottom of the North Island will be lashed by gale-force winds of up to 120 kilometres an hour, both today and tomorrow.

Forecasters are warning the bitterly cold southerly will spread north from late morning.

That'll bring gales to many coastal areas, and squally thunderstorms and snow in the south and east of the South Island.

Snow is expected to fall above 200 metres in the south and east of the South Island today, and possibly as low as 100 metres in parts of Canterbury tomorrow.

Large seas of up to 6 metres are tipped to hammer the South Island's coast from later today, and through to Friday for the North Island.

Official road warnings are starting to filter out.

The AA said snow chains are now needed on the Crown Range Road in the South Island, as well as at Coronet Peak Road.

It said extreme caution is needed.