Small tornadoes may develop today as heavy showers and severe thunderstorms are expected to hit the upper North Island, MetService says.
Forecaster Chris Noble said "one or two small tornadoes" may develop, "but if any do form they will only affect very localised areas".
The weather bureau has issued a severe thunderstorm watch for Northland, Auckland, Great Barrier Island and the Coromandel Peninsula, which is in place until midnight.
Heavy showers and isolated thunderstorms are predicted to sweep across Northland, working south across Auckland and the northern Coromandel later this evening, MetService said.
People are being urged to remain indoors with heavy rain bringing risks of flash flooding, especially about low-lying areas such as streams, rivers or narrow valleys, Mr Noble said.
"Driving conditions may also be hazardous with surface flooding and poor visibility in heavy rain.
"Additionally, these storms may be accompanied by strong wind gusts and hail."