Swimsuits may need to be swapped for ski jackets as a weather bomb looks set to wipe out the sweltering heatwave, even leaving parts of the South Island dowsed in snow.

The unseasonable storm is expected to bring snow to as low as 1000m in some areas just days after it reached 37.6C in Clyde - New Zealand's hottest January temperature in 14 years.

MetService today issued a swathe of heavy rain and wind warnings across all of the South Island and bottom of the North Island for the coming two days.

MetService says the storm will come from the Category One Tropical Cyclone Fehi, which formed near New Caledonia earlier on Monday, and will smash into the South Island on Thursday and early Friday bringing heavy rain and gale force winds up to 100km/h.

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The heaviest rain is expected in Westland and Fiordland where 200 to 400mm of rain could accumulate from tonight to early Friday.

Heavy rain is also expected for Mt Taranaki and Tararua Range.

The combination of strong winds, heavy swells and high tides could cause coastal inundation on the West Coast and Nelson on Thursday.

Snow is expected to fall on many of the South Island's skifields.

Cardrona Alpine Resort marketing manager Nadia Ellis said the changing weather was "crazy".

"It is so unseasonable. From the extremes of 35C in Wanaka earlier this week, to snow predicted, makes it all very interesting.

"At Mt Cook they are predicting greater snowfall this week than at Whistler Blackcomb in Canada, which is in the middle of winter."

While MetService is predicting 8cm of snow to fall on the Cardrona Skifield tomorrow, Ellis says the continuing warm weather means it is unlikely to settle enough for people to be able to get on their skis and snowboards.

Over the summer months Cardrona opens up to mountain bikes and carts, which Ellis expects could make for some "interesting rides" in the snow.