Transpower doesn't know how long it will take until they learn what caused yesterday's three-hour-long blackout across Northland.
A high-voltage circuit breaker tripped yesterday morning, leaving 90,000 people without power until midday.
It affected water supply, sewer systems and traffic lights.
The Marsden Point Oil Refinery also had to completely power down for safety reasons.
Alison Andrew is the Chief Executive of Transpower.
