Customer credit cards and loans went "missing" from the Commonwealth Bank's online and mobile banking platform for around 24 hours on Wednesday.
Complaints started flooding in early Wednesday morning that credit cards and loan balances had been wiped from customer accounts on CBA's Netbank app and website.
Balances for travel money cards and BPAY-based bill payments were also reported to have disappeared.
The platform was only displaying the amount of funds customers had in their savings accounts. Other users reported being unable to access Netbank entirely.
The bank also revealed merchant terminals - including its Albert device - had been affected by the issue.
It was unclear whether the issue would affect scheduled payments. CBA said it would assess any fees charged during the outage on a case-by-case basis.
The bank initially advised an ETA of midday for service restoration but later changed that advice to no ETA.
"For our customers impacted by this issue we are sorry, please be assured we are working on this as a matter of priority. We'll keep you updated as new information comes to light," the bank said in a statement.
Update 5/4/18 9:25am: CBA's Netbank status page was updated on Thursday - more than 24 hours after customers first complained of the issue - to say services were now "running smoothly".
It has refused to disclose the cause of the issue.
"Our teams have worked around the clock to fix the problem and NetBank, CommBank app and CommBiz services are restored," it stated.
"We can also confirm credit card payments and transfers have been processed and customers will start to see these reflected in their accounts.
"We fully understand this has caused inconvenience and that some customers may have incurred late fees. We sincerely apologise for the inconvenience and would like to thank customers for their patience."