Power has been restored to more than 90 per cent of customers hit by major outages across the North Island this afternoon.
Electricity was cut to homes and businesses throughout the North Island, including Auckland and Wellington, after a fault at the Huntly power station about 12.38pm.
Reserve power is being utilised until the problem is fixed.
Transpower, which operates the national grid, said the fault led to about 500 MW of power being disconnected to protect the North Island power transmission system.
Both Transpower and Genesis Energy, which owns the Huntly station, were yet to to determine the cause of the outage.
They were working to find the cause and restore supply, but it was not yet known when the station would begin generating power again.
In the meantime, reserve generation had been brought online to allow retailers to restore electricity to customers.
Transpower said power had been restored to 90 per cent of customers and companies were working hard to restore electricity to the rest.
"Technicians are working at Huntly to locate the cause of the outage. Huntly remains offline at this stage but there is sufficient reserve generation to meet demand.''
Auckland supplier Vector said most Auckland customers now had power after electricity was restored from about 1.15pm.
In the Wellington region, electricity has been restored to most customers after outages in the lower and central Hutt Valley, Petone, and suburbs including Hataitai and Karori.
In the central and lower North Island, about 30 per cent of Powerco's network was down, affecting parts of Manawatu, Wanganui and South Taranaki.
Power outages were also reported in Bay of Plenty, Gisborne and Hawke's Bay.