Transpower says a fault on Vector's equipment at a substation in west Auckland caused a power outage affecting about 46,000 customers on Thursday night.
The Fire Service said was called to the Hepburn Road substation in Glendene about 6.30pm after a power transformer exploded.
Electricity company Vector said crews have managed to restore power to customers in Henderson Valley and to many Rosebank and MacLeods Road area customers.
Crews are continuing to work on restoring power to other customers. Transpower said staff are on site assessing the damage - but it won't be a quick job.
