BusinessDay understands the matter is under police investigation, with CommBank issuing replacement cards from yesterday afternoon to about 8000 customers who might have been affected.

Do you know more? Email to: czappone@fairfax.com.au

Westpac said it was aware of the problem and had blocked a small number of cards as a result but that the data breach did not occur at the bank.

"There’s not been a compromise of security at Westpac but we are aware of an issue," a spokesman for the bank said.

NAB also confirmed that "a small number" of their accounts had been compromised. Bendigo Bank told BusinessDay it had cancelled and reissued 2300 cards.

CommBank discovered suspicious transactions on a number of credit card holders’ transactions and began cancelling the cards late yesterday. The bank pinpointed the merchant in common between the accounts and, as a precaution, cancelled the cards and reissued them, while notifying the bank that provides services to the merchant.

"The Commonwealth Bank continuously monitors all credit card transactions to protect our customers from fraud and during this process became aware of a potential credit card compromise," the bank told BusinessDay.

CommBank said that customers who received a message about potential fraud shouldn't assume that their account had been compromised.

"The Commonwealth Bank immediately alerted the Card Schemes MasterCard and Visa to the compromise and also the bank involved," CommBank said, declining to name the other bank.

Westpac-owned St George Bank also reported cancelling a small number of cards as a result of the security concern and said there had been no compromise of data.

"We have blocked a small number of cards as a result of a compromise affecting an Australian merchant," a spokesman for St George said, without detailing the number.

Both Westpac and St George said customers would not be liable for any fraud related to their accounts.

Rival National Australia Bank said its customers were not affected. ANZ said the merchant involved was not one of its clients.

Contact made

A spokesman for CommBank said it had "proactively contacted potentially impacted customers via SMS, email and letters to advise that their cards have been cancelled and will be reissued as a matter of priority". He declined to say where or when the breach occurred.

"In any instance where a Commonwealth Bank customer card has been compromised, the customer will be fully reimbursed and any fees refunded," he said.

Customer comments to this article offered some praise for CommBank's actions, although one respondent in Ballarat, Victoria, complained about difficulties in getting information after his card was cancelled.

"CBA texted last night to say my Visa card was cancelled and to ring for more details. Navigating the automated phone system led to a point where a messaged annouced they were busy taking calls and advised to try again later, then promptly hung up,'' the comment stated.

''Very poor customer service and quite frustrating. I got through to them today and they didn't say who the merchant that suffered the security breach was, but that it was not the Sony Playstation Network.''

Last month, Sony shut down its PlayStation Network after hackers had compromised the personal details of 100 million online users worldwide, including 1.5 million Australians.

In April, a data breach related to a contract firm with Dell Australia exposed thousands of customers to potential scams by hackers who might have used the information to misrepresent themselves as company representatives.

While the latest issue for Australian banks involves data security, they have been struggling with their own rash of technical difficulties in recent months as they try to upgrade ageing computer systems.

In March, a glitch tied up CommBank's ATM, eftpos and branch payment system for a morning, causing the bank's ATM system to permit customers to overdraw their accounts for a short period. The bank later had to track down customers to recoup the funds.

czappone@fairfax.com.au

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Follow BusinessDay on TwitterCommonwealth Bank discovered suspicious transactions on a number of credit card holders’ transactions. The bank found the merchant in common between accounts and, as a precaution, cancelled the customers’ cards and reissued them, while notifying the bank that provides services to the merchant.“The Commonwealth Bank continuously monitors all credit card transactions to protect our customers from fraud and during this process became aware of a potential credit card compromise,” it said.Any message about potential fraud risk doesn’t indicate that an individual’s account had been compromised, the bank said.