South Canterbury is under a "top priority" weather warning, with the risk of tornadoes about to strike Ashburton and Timaru along with severe thunderstorms for the South Canterbury region.

"At 03:22pm, MetService weather radar detected severe thunderstorms near Carew, Peel Forest and Arundel," it warns.

"These severe thunderstorms are moving towards the eastnortheast, and are expected to lie near Carew and Arundel at 03:52pm and near Carew and Lismore at 04:22pm.

"These thunderstorms are expected to be accompanied by very heavy rain, large hail and possible tornadoes."

Tornadoes can blow out windows, lift roofs, break large branches off trees, generate dangerous flying debris and blow vehicles off the road.