The outage, which a major Sydney daily reported to have also affected NSW Health Technology's Cumberland data centre, began at about 9am on 2 May.
The Health Service declined a request by iTnews to release details of the post-incident report.
It was also unable to confirm if the outage was caused by workers doing maintenance at the data centre, as was reported, or whether the failure of backup systems was grounds for a review of data centre systems statewide.
"A faulty circuit breaker caused a power outage, which affected Nepean Hospital's electronic medical record system," said Sydney West Area Health Service chief executive officer, Steven Boyages.
"Hospital staff immediately adopted the standard paper back-up system for patient records.
"Pathology and x-ray results were faxed to emergency departments and confirmed by telephone."
All computer systems were up and running again by 3pm, Professor Boyages said.
Patients needing emergency or urgent treatment were unaffected but "non-urgent patients unfortunately experienced some delays", he said.
It happened days after Health Minister John Della Bosca released the results of trials of electronic medical records, which the department will use in 188 hospitals statewide before the end of next year.
He described the $100 million project as a cornerstone of NSW Health's information and communication technology strategy, which is "modernising the way health services are supported in NSW".
"Prior to the introduction of (electronic medical records), some requests for medical imaging and pathology could require referral back to the requesting clinician due to incomplete or illegible hand-written records," Della Bosca said.
"This technology will improve the efficiency of hospital care and free up doctors and nurses to focus on patients and not paperwork which will further improve patient safety."