Transpower says maintenance contractors removed too many nuts at the bottom of three legs of the power pylon that collapsed last week.
Much of Northland lost power following the collapse - it was fully restored yesterday afternoon.
Transpower chief executive Alison Andrew said crews from Omexom were working on the pylon and doing "routine" maintenance.
Too many nuts were removed from the legs connecting to the base plates, something that was "unprecedented and inconceivable,” Andrew said.
She said there had been a failure to follow procedure and that the crew working on the tower had been stood down immediately.
"While we are very grateful that no one was hurt when the tower fell, the failure to follow procedure resulted in a significant power outage that had a real impact on the people of Northland."
An external party had been appointed by the state-owned enterprise to conduct an independent investigation.
WorkSafe is also conducting an investigation into the incident.
"We are committed to learning from this event and implementing any additional controls," she said.
Northlanders who were without power would be able to make a claim through the Consumers Guarantee Act.
Omexom NZ managing director Mornez Green also described the incident as "unprecedented".
"We removed too many nuts from the bolts that resulted in the tower falling over.
"Omexom promptly worked to restore the power.
"I can assure you we will be open and transparent in the investigation."
He said it was "very fortunate" that no one was killed and that the competence of the people on site would be assessed.
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