Capital and Coast District Health Board is trying to find out why back-up electricity supplies at Wellington hospital failed to immediately switch on during a power cut on Thursday afternoon.
A fault at Transpower's Wilton substation cut power to about 50,000 customers in the central city and outer suburbs for up to two hours.
The DHB says back-up generators failed to come on immediately when the power went off at 3.20pm.
However, intensive care services clinical director Peter Hicks says the hospital was able to maintain adequate power to keep essential life support services going.
Transpower says it is investigating the cause of the blackout. Chief executive Patrick Strange says they have some idea of what happened, but need more answers.