The electricity supply has been restored to Wellington Hospital.
A failure occurred in the grid around the hospital just before 11am on Friday, but power was instantly restored by batteries before back-up generators kicked in.
There was no disruption to emergency services or the five operations under way at the time. Mains power gradually returned from about 11.30 on.
Wellington Electricity, the company that manages the hospital's supply, says it is investigating the cause of the failure.
Company spokesperson Ryan Auger says the failure was isolated to the hospital and power was restored just after midday.