The Orion Energy Company said 70% of Christchurch households - about 150,000 customers - were still without power as night fell following Tuesday's earthquake.
Power was cut to most of the city following the 6.3-magnitude earthquake, which struck at 12.51pm.
Orion said about 80% of its customers in Christchurch and rural Canterbury initially lost power, but supplies had been restored to some households in Banks Peninsula, Akaroa and parts of western Christchurch.
Chief executive Roger Sutton said it was not possible to say when power would be reconnected, and it was proving very difficult for linesmen to move around the city.
He said on Tuesday that it would be dangerous for Orion workers to attempt to reconnect power to the central business district.
He said if they were to restore power remotely to a damaged plant, there was a risk of damaging it further and of setting fire to any nearby houses that had gas leaks.
However, Mr Sutton said damage to the city's power infrastructure did not appear to be too serious.