The cause of the blackout was traced back to the Ōtāhuhu sub-station, the city's main transmission switching station.
[3][4] Investigation of this incident found that maintenance of the electricity transmission system was not adequate and that this substation had major and minor design deficiencies.
At approximately 14:45, power was restored to most of Auckland, except Penrose, Glen Innes, East Tāmaki, and Ōtāhuhu.
These include the diversity provided as part of the North Island Grid Upgrade Project by connecting the new Whakamaru to Brownhill Road transmission line to Pakuranga substation, rather than directly to Ōtāhuhu.
A second major project, the North Auckland and Northland grid upgrade project provides underground 220 kV cables from Pakuranga to Penrose, and from Penrose to Albany on the North Shore, via Hobson Street in the Auckland CBD.