A severe thunderstorm watch has been issued for the eastern North and South Islands - with flash flooding and small tornados possible

Slow-moving isolated thunderstorms are expected through inland Rotorua, Bay of Plenty, Gisborne/Tairawhiti and Hawke's Bay - until 10pm.

Wild weather's predicted for Canterbury south of Rangitata River, North Otago, Dunedin and northern Clutha - until 9pm.

MetService says some thunderstorms may be severe - with up to 40 millimetres rain, and hail larger than 10-cent coins.

Dunedin's under a severe thunderstorm warning, and being pelted with torrential rain and hail.