There are reports of heavy crop damage from severe hail storms in parts of Canterbury early on Monday afternoon.
Thunderstorms caused hundreds of lightning strikes and produced hail stones up to 3cm in diameter.
Federated Farmers says initial damage reports suggest the worst hit farms could lose between 20% - 40% of their wheat, barley and clover crops.
It says softer crops like peas and brassica would have been shredded.
MetService says more thunderstorms are expected around the upper South Island and in Hawke's Bay and Gisborne on Tuesday afternoon, but they are not likely to be severe.