The MetService is warning that flooding and one or two small tornadoes could hit the upper North Island on Friday night due to heavy thunderstorms.
Heavy rain of up to 25 millimetres an hour is forecast for Northland, Auckland and Coromandel.
MetService says that could cause flash floods in low-lying areas, which may result in slips.
It also warns that small tornadoes are possible.
The bad weather will mean reduced visibility for motorists and they are urged to take care.