South Island farmers are being warned to prepare as one of the biggest polar blasts of the year edges closer.

WeatherWatch.co.nz head forecaster Philip Duncan said a southerly storm would arrive early next week from the Antarctic.

"This looks to be one of the more significant southerlies of 2018, at least for the lower part of New Zealand.

"At this stage Monday to Thursday next week looks wintry in the lower half of the South Island and while there will be some colder, windier, rougher weather in other parts of the country, it's truly the lower portion of the South Island most exposed to a brief return to winter conditions."

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Southland and Otago were likely to be hardest hit with snow flurries potentially settling down to 200m, and even towards sea level in isolated flurries.

The main concern was for farmers with newborn stock, Duncan said.

"Snowstorms in September and October are incredibly normal, but the timing is extremely poor for growers and farmers.

"Daytime highs will be miserable along with wintry, damp, southerlies.

"Whether it's snow flurries or just wintry rain showers the reality is that this will be a miserable week for livestock in the lower South Island, especially newborn lambs, with daytime highs next Wednesday ranging from just 3C to 9C."

Such a mild winter and start to spring meant soil temperatures were also fairly mild, which would help melt the snow faster at lower levels, Duncan said.

Overall next week was in for a bit of rain, snow, rain showers and strong, cold, winds from the southwest.

Snow was also possible in Queenstown, Wanaka, Lumsden and potentially Dunedin's hilltop suburbs.

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"The equinox is this Sunday which means no matter how you slice it and dice it, next week is officially spring - but winter is reminding us it has only just come to an end and summer is still a few months away," Duncan said.

"We must stay on our toes for another month at least."