Staff and patients at Christchurch hospital were left in the dark for an hour last night after the power went off at around 8.30pm and the back up generators didn't automatically connect to the hospital's systems.

Battery-powered lighting was brought back from when the hospital had no electricity during the quakes, and some staff scrambled for torches or grabbed pens and paper to record notes

Some staff even reverted back-to-basics to do patient observations, such as taking a patient’s pulse manually, said Te Whatu Ora.

"There was some anxiety obviously but the nursing staff in particular were able to calm people down... it was a bit of excitement really, it was like when you get a power cut when you're a kid," clinical nurse manager David Smythe said.

"We saw security come round a few times to make sure we were OK as well as the senior members around through ED next to us," said Shannon Burrows from the children's emergency department.

Te Whatu Ora says a network power outage is to blame — and is investigating why the generators worked for Christchurch Women's Hospital whereas in other areas they failed to connect.

According to Te Whatu Ora, at the time there were "a total of 478 patients in Christchurch Hospital, including 96 in ED" and "all were well supported and cared for".

"There was one patient whose surgery was safely completed just as the power cut hit. In addition, another patient was about to have a procedure which was deferred until after the power came back on," Te Whatu Ora's Lisa Blacker said.