An explosion at a sub-station at Bombay, south of Auckland, cut power to 9,000 homes on Friday afternoon.
Transpower says it happened at 12:06pm, with power restored about 3.30pm.
Transpower says no one hurt in the explosion but equipment was badly damaged.
It says repair crews will be working through the night.
Transpower says electricity is limited and is advising residents in the Mangatawhiri area to conserve power between 5pm and 7pm to avoid further cuts.