Thunderstorms, hail and high winds are expected in Northland, Auckland and parts of Coromandel Peninsular this afternoon.
MetService said there was also a small risk of tornadoes or waterspouts forming.
The thunderstorms may be severe, with wind gusts exceeding 110km/h and hail larger than 20mm.
The storms were expected to move onto Northland and western suburbs of Auckland within the next few hours, MetService spokesman Dan Corbett said.
"The radar is lighting up like a Christmas tree already to the west of Auckland and Northland," he said.
Showers would become more heavy, and squally winds would increase through the afternoon, with the peak risk for thunderstorms from about 1pm through to 6pm or 7pm tonight.
Mr Corbett said they could not completely rule out the risk of a small tornado or water spout. "They will be short lived but it's always something to be aware of."
A severe thunderstorm watch was in place for Auckland, Northland and the northern Coromandel Peninsular, which would be updated to warning as individual storm cells formed.
Auckland and Northland were at highest risk for severe storms, but thunderstorms were also expected in Waikato, Bay of Plenty and Taranaki, and western and northern parts of the South Island.
Weather was expected to improve by the end of the week.