A power outage in the early hours of Wednesday morning affected 20,000 customers in Top Energy's southern network - including Rawene, Russell, Kerikeri, Paihia, Opua and Kaikohe, as well as a section of its northern network in Kaitaia.

Top Energy spokeswoman Philippa White said it was likely to have been caused by a fault on the 33kV circuit breaker at the Kaikohe substation. The fault caused an automatic safety tripping of two transformers and shut down the power supply between 2.45am and 6.20am.

Ms White said because communications (internet and cellphone networks) were also affected crews had to manually inspect equipment at the substation and redirect the supply of electricity before power could be restored. Further investigations are under way to understand what caused the tripping to occur.

After a day-time power cut last October that affected 30,000 customers in the region and forced a number of businesses to close, Top Energy warned of similar occurrences in the future. On that occasion the cause was traced to a broken insulator in the electricity substation in Kaikohe.

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Because the latest power cut occurred at a time when most people were in bed the impact on private households and businesses was not as severe this time.