People are encouraged to take shelter as the storm approaches, preferably indoors and away from windows, and secure loose objects around their properties.
People should also avoid streams or drains and watch out for fallen trees.
The risk of thunderstorms is considered high from southern Auckland through the central high country, and to inland Hawke's Bay and the ranges of Gisborne.
The Far North will get thunderstorms this morning and early afternoon, but that is expected to ease.
Northland's watch is in place until 2pm, with localised downpours of 25mm to 40mm possible.
MetService says there is a moderate risk that thunderstorms may become severe with localised downpours of 25mm to 40mm an hour, and any thunderstorms could produce localised wind gusts of 90km/h and hail of 10mm to 20mm in diameter.
Rainfall of this intensity can cause surface or flash flooding, especially in low-lying areas such as streams, rivers or narrow valleys. Driving in these conditions will be hazardous.
Other parts of the North Island are at moderate risk but that may potentially become severe.
These parts of the North Island include: Great Barrier Island, Coromandel Peninsula, Waikato, Waitomo, Taumarunui, Bay of Plenty, Rotorua, Taupo, Gisborne, Hawke's Bay and Taihape.
A low risk of thunderstorms covers Taranaki, areas near the coast of Whanganui and Manawatu, and southeastwards to Wairarapa.
For the South Island, showers are expected to develop in Canterbury and eastern Otago.
There is a moderate risk for North Otago and South Canterbury this afternoon and evening, as these showers or possible thunderstorms may deliver small hail and brief heavy rain.
At night, a front reaches southern Fiordland, bringing a moderate risk of thunderstorms which may deliver heavy rain of 10 to 25mm/h.
MetService says no thunderstorms or significant convection are expected elsewhere.
