The Cook Strait power cable is fully operational again. Supply was disrupted on Monday night.
During the outage, which affected the cable known as Pole Two, emergency power was delivered from the South Island to the North Island via the other inter-island cable, Pole One.
Transpower chief executive Patrick Strange says Pole Two was restored at 1am on Thursday.
He says the outage was caused by a complex circuit fault in a component of a transformer at Benmore.
Dr Strange says the back-up cable, Pole 1, operated perfectly during the outage, due to extensive maintenance work over the past six months.
Energy Minister Gerry Brownlee earlier described the failure as alarming and said the transmission infrastructure has been neglected for too long.
