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.