Power to 1600 homes in and around Lyttelton has been out since about 3am, and may not be fixed until 8pm.
Electricity company Orion said a lines fault caused the cut.
More than 1600 properties are without power.
An overhead line failure cut power at 3.20am to Cass Bay, Corsair Bay, Governors Bay, Lyttelton, Pigeon Bay and Rapaki.
Orion general manager John O'Donnell said because the faulty line was in difficult mountain terrain it could take until 8pm to fix.
"It's very steep, it's on the Lyttelton side so it's on the Port Hills. So it is steep country and it's quite difficult terrain."