Power was cut to 1000 Oamaru homes after two power poles fell in Beach Rd early on Saturday.
One of the wooden poles is believed to have rotted about 30cm below ground, causing it to fall.
It took with it another pole and a transformer.
Traffic was diverted along Roxby and Traill Sts after the incident about 5.40am.
Network Waitaki chief executive Graham Clark said it was not always possible to accurately assess the underground condition of wooden poles.
''We'd rather it didn't happen ... we're always looking for ways to improve testing our poles.''
Power was restored to 80% of homes within about 20 minutes, Mr Clark said.
However, some Beach Rd residents were without power until late afternoon.
Network Waitaki had a large team working on the problem for most of the day.
Both poles had been replaced and the road was reopened about 5pm on Saturday, Mr Clark said.
The condition of all poles on the Network Waitaki network were checked as part of a five-year inspection, he said.
''But you can only test what you can see.
''We're always looking at ways of testing underground.''
