The Commonwealth Bank has warned customers of delays at branches and call centres on Friday as they work through the "backlog" caused by a major outage on Thursday.

Many were left stranded after an IT issue meant CBA customers were unable to transfer money for more than six hours. An estimated eight million people were affected by the outage that forced some branches to close early.

“We have been working to restore the network systems overnight and these are now returning to normal," a spokesman for the CBA said on Friday. "We are working through the backlog as quickly as possible, but rest assured that all payments both into and out of customer accounts will be properly processed."

Branches will open as normal on Friday, but the CBA said they were predicting delays across the board.