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Gales and huge ocean swells are expected to batter much of New Zealand from the west over the next few days.

The source of the severe weather is an enormous, deep low-pressure system churning far to the south of Invercargill and sending a series of fronts sweeping across the country.

MetService weather ambassador Bob McDavitt said yesterday that the Roaring Forties, a band of strong westerly winds, were expected to move temporarily north into the Tasman Sea.

"So we are going to have some wild and windy weather over the next few days. Particularly the west coasts [of the country's islands] are likely to get humungous swells from the Roaring Forties as they move in, particularly during Friday and Saturday."

Philip Duncan, of the WeatherWatch website, said the low was currently one of the largest storms on the planet, "covering an area the size of Australia" and with a central pressure that had sunk to around 950 hectopascals.

MetService chief forecaster Peter Kreft said a gradual change to bitterly cold conditions was likely to spread on to the south from late tomorrow and this might bring snow to low levels.

Near-zero wind-chill conditions were likely to cause stress to vulnerable farm animals.

The MetService has issued severe weather warnings for many western and alpine areas of the South Island.

The fronts should dump heavy rain and significant snowfalls in Fiordland and the Southern Alps by tomorrow, with heavy snow expected on the Milford Track and other tramping tracks.

There is a 40 per cent chance of gales becoming severe in Wellington, Wairarapa and the Marlborough Sounds and a 20 per cent chance of heavy snowfalls on the central North Island hill country.

Snow will fall as low as 600m on Mt Ruapehu, but with severe gales that typically close the commercial skifields.

The upper North Island is generally not expected to fare so badly, although rain or showers are predicted in most areas, with heavy falls for some, and Aucklanders may have to brace for galeforce winds.