Air traffic control agency Airways says a contractor hired by Auckland International Airport was responsible for the failure of runway lights last November.
Thousands of passengers were disrupted and 22 flights were diverted or cancelled when the lights stopped working.
Airways originally thought that lighting cables may have been damaged by lightning strikes.
But technical manager Peter O'Regan says during the runway widening project in 2006 a contractor placed a duct in the wrong place.
Mr O'Regan says the duct moved one of the lighting cables and caused it to erode and eventually short-circuit over time.
