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Network Waitaki engineering manager Brad Fleming said areas from Shag Point in the south to Waitaki Bridge and inland to Duntroon were affected when the power supply to the Waitaki electricity distribution company was disrupted.
Mr Fleming said Network Waitaki was able to resupply Oamaru Hospital, some schools, supermarkets and emergency services with power within half an hour.
"Network Waitaki was able to resupply essential services by taking supply from their Lake Waitaki source ... to resupply some of the Oamaru area," he said.
"We have a Lake Waitaki grid exit point as well; within Network Waitaki's network we were able to take supply from up there and push it as far down to Oamaru to keep the essential [services operating]. You can't resupply everyone.
"As soon as the issue happened, we took charge of our internal network to try to supply as many people as possible."
Transpower NZ said on its Facebook page that one circuit was out for maintenance and "unfortunately the second circuit tripped".
A helicopter picked up the staff member who was working on the maintenance programme, a Transpower NZ spokeswoman said.
"They then flew over the length of the line as a visual inspection to see if they could identify the cause of the trip which occurred on the wire (conductor) that is the highest one on the tower.
"There was no visual indication of cause so the most likely cause was a bird (given the height of the wire)."
Transpower NZ was able to restore the line that tripped faster than reinstating the line that was out for maintenance.
The company put the second circuit back in overnight but would take it out of service again today to complete the maintenance.
Power was out from about 12.25pm until about 3pm.