Equipment failure at the Kaikohe substation was suspected but initial investigations by Top Energy crews on site showed no apparent issues.
Instead, it appeared a fault has occurred on the 33 kV line to Mt Pokaka.
“Unfortunately, the primary protection didn’t operate as expected which would have limited the outage to a localised area, so backup protection tripped the station, causing a wider outage,” Top Energy said.
“We will be investigating further, and we apologise for the inconvenience this has caused on a Friday afternoon heading into the weekend.”
Hato Hone St John said the outage had not impacted its operations.
Health NZ confirmed generators at the Kaitāia and Bay of Islands hospitals had kicked in, meaning they were able to operate as normal.
The Northern Advocate has contacted other emergency services to see whether they are impacted.
More to come.