An explosion at Northpower's Maungatapere station has cut power to thousands of homes and businesses.

Spokesman Steve MacMillan says the explosion happened at about 11.30 this morning, immediately cutting power in the Maungatapere, Poroti, Kioraroa Road and Whangarei South area - including part of the CBD.

He says many properties will have had power reinstated within minutes, but work is underway to get the rest reconnected as soon as possible.

It's not yet known what caused the explosion.