Hail stones up to about 3cm in diameter have ripped across mid-Canterbury, smashing windows and damaging crops.

A southerly storm working its way up the South Island dropped hail on Mayfield, near Ashburton, for about 50 minutes.

Several locals have told The Press the hailstorm broke many windows at homes and businesses in the township.

Local crop farmer Rab McDowell predicted about 40 per cent of his carrot and maize crops were a ''right off'', but he would know more in the morning.

''I haven't seen hail through here that broke windows before.''

The large hail was expected, as earlier a severe weather warning was issued for mid-Canterbury which included hail, torrential rain and possible tornadoes expected.

Metservice forecast severe thunderstorms for the Mt Somers, Mayfield, Lismore, Westerfield and Ashburton Forks areas just before 6pm today.

The strong storms were expected to move across Methven, Winchmore and Lauriston about 6.20pm.

Metservice warned that torrential rain could cause surface or flash flooding about streams, gullies and urban areas, making driving conditions extremely hazardous.

Large hail could cause significant damage to crops, orchards, glasshouses and vehicles, also making driving conditions hazardous.

Tornadoes could blow out windows, lift roofs, break branches off trees and generate dangerous flying debris and blow vehicles off roads.

The Ministry of Civil Defence and Emergency Management advises people to take shelter as storms approach.

Other tips include moving cars under cover or away from trees, securing any loose objects around properties, checking drains and gutters are clear, and being ready to slow down or stop if driving.

People should also be aware of fallen trees and power lines after the storm has passed, and should avoid streams and drains in case of flash flooding.

Source - http://www.stuff.co.nz/