Humid and unstable conditions are expected to bring thunderstorms to many areas of the North Island during today, producing localised heavy rain and hail, says a MetService spokesperson.

"In the Bay of Plenty, including Rotorua, Waikato and the Coromandel Peninsula, there is a moderate risk some of the thunderstorms could become severe," says a MetService spokesperson.

"These severe thunderstorms could bring large hail greater than 20mm in diameter, strong winds gusting greater than 110 km per hour, and even the risk of a small tornado."

The MetService is warning that large hail can cause significant damage to crops, orchards, vines, glasshouses and vehicles, as well as make driving conditions hazardous.

Wind gusts greater than 110 km per hour in strength can cause some structural damage, including trees and power lines, and may make driving hazardous.

"If any tornadoes occur, they will only affect very localised areas."

MetService notes at a Severe Thunderstorm Watch means conditions are favourable for severe thunderstorms in and close to the watch area. People in these areas should be on the lookout for threatening weather conditions and monitor for possible Severe Thunderstorm Warnings.

For information on preparing for and keeping safe during a storm, see the Civil Defence Get Ready website.