Severe thunderstorms may hit Dunedin, North Otago and Clutha this afternoon bringing heavy rain and hail as large as 20mm to some areas, MetService warns.

It has issued a severe thunderstorm warning until 10pm tonight as conditions have become favourable for such storms, Metservice forecaster Paul Mallinson said.

"Daytime heating and wind convergences are expected to trigger showers and scattered thunderstorms over Otago and Southland.''

Some of those storms might bring hail, possibly exceeding 20mm in diameter, he said.

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"Large hail can cause significant damage to crops, orchards, vines, glasshouses and vehicles, as well as make driving conditions hazardous.''