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Motorists are being urged to take extra care as a ''very cold'' airstream hits the South, bringing showers with hail and snow.
MetService forecaster Heath Gullery said last night there may be some snow overnight but the worst weather was expected after 6am.
It was most likely to be developing this morning, then continue to the end of today or early tomorrow, he said.
Snow was expected to 300m across Southland and Otago this evening, while tomorrow Dunedin's forecast is for showers with hail and strong cold southwesterlies.
NZTA Otago-Southland journey manager Graeme Hall said June to August was ''traditionally the most dangerous time for driving on roads in the Otago-Southland region''.
Yet many motorists did not take the time to prepare for their journey.
Mr Hall called on motorists to be ready for the weather.
''We need everyone to drive to the conditions, to allow more time for their trip, be prepared for unexpected hazards and ensure their vehicle is winter fit, as crashes can result in more serious consequences.''
The margin for error on wet slippery roads was ''tiny'', he said.
Anyone travelling during the winter months needed to prepare by getting their tyres, brakes, windscreen wipers, lights and steering system checked.
''Most important, though, is to plan your journey.
''Check the weather forecast; consider if you really need to travel when heavy rain, ice and snow are forecast; check road conditions and take the safest route - even if it is the longest.''