Weather: Thunderstorm threat looms across much of New Zealand

Much of New Zealand faces the threat of thunderstorms on Wednesday, with the risk stretching from the bottom of the South Island to the top of the North Island.

"High temperatures and humidity at the surface, along with cooler air aloft and sea breeze convergences, mean there will be a few showers around the North Island," MetService warns.

"In some areas, the ingredients are just right to get thunderstorms, and some of those could be severe with localised downpours."

In the South Island, there is a low risk of thunderstorms in Buller, Westland, Otago and Southland from the afternoon onwards.

There is a also a moderate risk of thunderstorms this afternoon and evening around Dunedin and North Otago. If they do happen, MetService says they will come with brief heavy rain and small hail.

In the north of the North Island, there is a moderate risk of a few thunderstorms about eastern Northland this afternoon and evening, and low risk covering other parts of Northland and northern Auckland.

In the central North Island, there is a moderate risk of thunderstorms about Gisborne, Hawke's Bay, the eastern Bay of Plenty ranges and the central high country.

A severe thunderstorm watch has been issued for Gisborne and Hawke's Bay this afternoon and evening.

"Between about 2pm and 9pm today, some of these thunderstorms are likely to be severe with rainfall rates of 20-40mm/hr," MetService warns.

"Rainfall of this intensity can cause surface and/or flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys.

"Driving conditions will also be hazardous with surface flooding and poor visibility in heavy rain."