WeatherWatch NZ has reported "dramatic downpours" passing over Timaru and Oamaru, with one downpour blanketing Timaru in thick and heavy hail.
Hail stones as large as golf ball were seen across the region and there had been confirmed reports of broken windows and roofing from the large hailstorm.
Meteorology provider MetService has issued a severe thunderstorm warning for Selwyn and Ashburton, which are expected to be accompanied by large hail and possible tornadoes.
Tornadoes could blow out windows, lift roofs, break large branches off trees, generate dangerous flying debris and blow vehicles off the road, MetService said.
Forecasts suggest there could be more tornadoes and golf-ball-sized hail for the South Island's east coast.
"The downpours may contain thunder as well, but should ease this afternoon," WeatherWatch NZ said.