An investigation into the substation fire which cut power to tens of thousands of Aucklanders will be jointly carried out by Transpower and Vector.

In a statement today, the companies said they would be co-operating fully and providing all assistance possible to the inquiry that Energy Minister Simon Bridges requested the Electricity Authority to carry out.

The investigation would be done by independent industry experts appointed separately by Transpower and Vector along with an overseas cable expert.

The Fire Service would also provide input on the forensic analysis of the fire.

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The investigation would cover:

* The cause of the event on October 5

* The extent and impact

* Risk mitigation and whether those mechanisms operated as they were meant to

* What could be learned from the event and recommendation to prevent similar events