An Electrical Authority report into the Penrose substation fire last year has found the incident was caused by the electrical failure of a Vector transition cable.
The blaze on October 5 caused a major outage across large parts of Auckland, with Transpower chief executive Alison Andrew, along with Vector's chief executive Simon Mackenzie, issuing an apology to those affected.
Both Transpower and Vector say they have been reviewing what caused the fire with the help of an international cable expert and have identified ways to ensure it doesn’t occur again.
Ms Andrew says Transpower has checked other critical sites with a similar set-up and is confident the sites aren't at risk.
"We have around 169 substation sites around the country and a few of these have similar criticality to Penrose. Risk assessments have been undertaken at these sites and we are confident that the assets are in good operational order. We are systematically working through our other substations with our customers."
In a statement, Energy and Resources Minister Simon Bridges says he welcomes the report and is satisfied with the steps both companies have taken during the investigation.
"I’m pleased the inquiry has found that, during the outage, Vector and Transpower communicated effectively and every effort was made to get the electricity back up and running as soon as was practicable."
Mr Bridges says the issue needs to be seen as a one-off incident, and that New Zealand's power system is of a world-class standard.
"The outage needs to be seen in the context of a world-class system with strong security of supply. This Government has made much-needed investment to ensure the lights stay on, including a multi-billion programme to upgrade transmission infrastructure."