Around 42,000 St George bank customers have had their account details sent to other people.
Other people's bank account details have been sent to around 42,000 St George bank customers.
Account numbers and transaction histories were wrongly sent on printed statements, BusinessDay reports.
The bank which offers St George savings accounts among others blamed the "production issue" on an external printing company.
One Killara, New South Wales-based customer was concerned that someone else had received their details and was asked for reassurances from the bank in writing. Nevertheless, it was explained to them "this would not be forthcoming".
The bank maintains customers are still able to access their own details on the phone, online or by visiting branches.
A spokeswoman said: "Once the investigation is complete, we will work with our external supplier to ensure that this issue does not occur again."
Security fears also occurred last month when Martin Pakula told the Herald Sun that the Victorian Taxi Directorate often receives grievances about credit card fraud in taxis.