The Auckland District Health Board says a staff member will sit with emergency generators throughout the weekend in case the main electricity supply fails.
The back-up generators at Auckland City and Starship Children's Hospital failed during routine testing on Wednesday morning.
The DHB says the problem was with a system supposed to automatically start the generators if the power fails.
Spokesperson Richard Aickin says the hospitals are running as normal on mains power and there is no risk to patients. However, the DHB is taking precautionary steps.
Dr Aickin says a staff member will sit with the generators so they can manually start them if the power fails.
The DHB says patients who missed out on elective surgery may have to wait up to a week for treatment.
Dr Aickin says seven people were able to have minor surgery using mains power.
However, 49 procedures have been delayed. They include heart operations which are too complex to perform without reliable back-up power.
Dr Aickin says acute cases will take priority but he hopes all patients will be seen within a week.