Thunderstorms are forecast across the upper North Island on Tuesday night and early on Wednesday.
MetService says an active front is moving across Northland and Taranaki about midnight, moving quickly to the east of the Coromandel Peninsula.
Squally thunderstorms could become severe, with potentially wind gusts of 120km/h or more.
There is also the possibility that one or two of the storms could produce tornadoes - especially around the coast.
The warning covers Northland, Auckland, Great Barrier Island, Coromandel Peninsula, Waikato, Waitomo, Taumarunui