More than 28,000 Northpower customers were left in the dark on Saturday when their power suddenly shut off.
A fault on high voltage equipment at the region's largest Northpower substation in Kensington caused a power outage around 4pm around Whangārei and up to 70km away in Bland Bay.
Northpower Network general manager Josie Boyd said the fault with the 33kV busbar - a metal bar with high levels of current inside switchboards - resulted in an impact larger and outage longer than they were used to dealing with.
She apologised on behalf of the electricity distribution company for the inconvenience and said the company was still investigating the cause of the fault.
Around 8000 customers in Kamo and Hikurangi had to wait nearly six hours for power to be restored.
Rural areas affected by the outage were included Ngunguru, Parua Bay, Kauri, Hikurangi, Hukerunui, and all East Coast locations from Whangārei Heads to Bland Bay.
However, electricity was returned to most other people within an hour after power was switched from other parts of Northpower's electricity network while the substation was repaired.
Power had been restored to all homes and businesses by midnight.
Customers in Whangārei city, Kensington, Whau Valley and Tikipunga - fortunate enough to have their power returned earlier - were asked to conserve power to manage higher than usual loads experienced by some parts of the network, Boyd said.
"People did a great job of conserving enough energy to help us keep the power on where we could, so we thank them for that."
Northpower operates and maintains both the electricity distribution and fibre networks in the Whangārei and Kaipara region with around 60,000 connected customers.
With a huge portion of their customers affected by the unexpected power outage, Boyd said staff worked incredibly hard to get the power back on.