A former tropical cyclone is just 24 hours from smashing into the South Island, bringing torrential rain, destructive gales and storm surges that threaten to flood coastal communities - and even some snow.

The first weather warnings have been issued, including a heavy rain warning for Westland and Fiordland.

The rain is expected to reach up to 380mm in parts of Westland south of Otira from midnight tomorrow, and to last until Friday morning.

Remarkably, snow may even fall in southern regions now baking in a record heatwave.

MetService said the rain was expected to start falling tomorrow morning and get heavier throughout the day. Up to 300mm is expected to drench north of George Sound.

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Those in the south are being warned to watch for fast rising rivers and streams and motorists are being told to be wary of flooding and slips in perilous driving conditions.

The major storm, which is expected to make landfall on the country's southwest coastline on Wednesday evening, is expected to cross the South Island Thursday and move away from the country Friday.

The Canterbury High Country, Otago, and Southland will also be saturated. Potentially destructive winds are also expected to lash west and northern regions. A strong wind watch is in force for Westland and Buller from early Thursday morning and lasts for a day.

MetService is warning the combination of strong winds and high tides may cause coastal flooding. West Coast and Nelson and are under threat Thursday.

Fire and Emergency New Zealand was also warning extreme temperatures when paired with wind on Wednesday could cause fire indices to spike to the extreme over the North Canterbury region.

All fires would have a rapid onset and a quick growth to "steady state", according to a rural fire updated issued by Fire and Emergency New Zealand.

There were restrictions in place for lawn mowing and operating machinery - these would remain in place until there had been a significant amount of rain or temperatures had dropped.

By the time Tropical Cyclone Fehi had New Zealand in its sights it would no longer be a cyclone but would still certainly pack a punch as it crossed the South Island.

Temperatures would take a dive as a cold southerly change swept through Friday, when Alexandra will reach just 14C days after it hit 35C.

MetService says there is even a chance of the region waking up to a dusting of snow on southern peaks Friday morning.

In the North Island a front was expected to bring heavy rain and gale-force winds across the western and central regions.

Weatherwatch.co.nz said the approaching storm would hit the South Island at the same time as a cold front from the Southern Ocean reached the far south. The storms would collide, bringing very heavy rain that threatened to flood Fiordland.

The forecaster said the bad weather would sweep north, bringing torrential rain, thunderstorms and flash flooding to isolated parts of the North Island.

Higher-than-usual king tides coupled with storm surge, strong winds and heavy rain put the West Coast at risk of coastal flooding.