MetService has released a Severe Thunderstorm Watch for a large part of the North Island.

Northland, Auckland, Coromandel Peninsula, Waikato, Waitomo, Taumarunui, Bay of Plenty, Rotorua and Taupo are affected.

Forecasters warn of localised downpours of 25 to 40mm/hr overnight.

MetService has said rainfall of this intensity can cause flash flooding, especially in low-lying areas around streams, rivers and in narrow valleys, and may also lead to slips and hazardous driving conditions.

More than 1,000 Aucklanders are without electricity tonight, and a Vector spokeswoman said sudden rain on hot power lines is to blame.

Around 1400 homes in Sunnyvale are off the grid because of a feeder fault at around 7.30pm.

Another feeder fault has also cut power to around 180 homes in Kaukapakapa.

Both areas should have their power back on by around 10pm.

Lines are down in near Three Kings and Hillsborough, but no homes have been affected.