The first customer complaints about instability in online banking services began surfacing on social media just after 8:30 am.
From CBA customer comments on Twitter, it appears that Web and phone banking, ATMs, Eftpos and even some in-branch services were down or experiencing serious technical problems for many users.
Some customers complained of being left unable to pay for transactions at petrol stations or shops.
The bank tweeted just after 1:30pm to say that all the problems had been fixed.
"All services are restored, including ATMs, NetBank and eftpos," it said, though cause of the problems wasn't explained.
The bank would not confirm how many customers were affected by the problem, which it described as a "technical issue". "We sincerely apologise to any customer who has been inconvenienced with this disruption to services," a spokeswoman said.