Clients who tried to access the NetBank service this morning told ZDNet Australia of repeated delays in connecting to the site.
A Commonwealth Bank spokesperson confirmed its online banking facility was down since (AEST) 8:40am today.
"It's been available on an intermittent basis ... some customers are getting through.
"But since this morning there's been a large customer demand that's built up and we're working to resolve that," he told ZDNet Australia.
He expressed hope that NetBank would be available by early afternoon, but could not provide an exact time for service to be fully restored.
The bank has yet to determine why the problem had occurred, he added.
The incident is the latest in a string of glitches since NetBank's AU$100 million upgrade in April.