Transpower says it believes a maintenance crew was responsible for the fall of a power pylon in Glorit on Thursday, which caused a mass power cut across the entire Northland region.
At a press conference on Monday afternoon, Transpower chief executive Alison Andrew said all the nuts securing the pylon to its baseplate on three legs were removed, "which caused the tower to lift off the baseplate and fall".
Andrew said it was the organisation's belief that "the specifications and procedures for this type of work were not followed".
Transpower has appointed an external party to undertake a full investigation into the incident.
The maintenance work was being done by contractor Omexom, which oversees transmission line work in the upper north of the North Island and the south of the South Island.
Managing director Mornez Green said the incident occurred while workers were cleaning and treating the nuts.
Green apologised to those impacted by the incident and promised the company would be "open and transparent" with investigators.
Thousands of homes and businesses lost power when the pylon fell on Thursday. Limited service was restored the following day, with full restoration completed on Sunday.
However, Andrew said there is still complex work to do to make the supply fully secure.
It is hoped this work will be completed by Matariki weekend.
Andrew and Green defended criticism that they didn't confirm the cause of the tower fall earlier, saying their immediate focus was to ensure everybody was safe and restore power.
"It was a complicated restoration… we did not want to take the focus away from all those people [working on it]."
