A security breach at one business's credit card payment terminal has seen thousands of cards cancelled by the major banks.
St George Bank said a merchant's credit card payment terminal in a business was compromised, but declined to reveal which merchant operated the terminal or which bank was associated with the merchant.
"It was a compromise that affected a business. It was a compromise at the business's end," a St George spokesman told AAP.
"We don't comment on the name of the merchant or their bank."
St George has so far blocked "a genuinely small" number of its customers' credit cards as a result of the security breach, and continues to monitor the remainder for any possible fraudulent activity.
Commonwealth Bank took more drastic steps, advising 8000 of its credit card customers of the potential breach, and reissuing cards "as a matter of priority".
"The bank continuously monitors all credit card transactions to protect our customers from fraud, and during this process we became aware of a potential credit card compromise through an Australian merchant acquired by another bank," a Commonwealth spokesman said.
Westpac and National Australia Bank said they had each cancelled a small number of credit cards, while ANZ said it was monitoring the situation through its regular anti-fraud measures.
All four major banks and St George said their own security systems had not been compromised.
"There's been no compromise of Westpac's security or systems," said a spokeswoman for Westpac, which owns St George.
All of the banks said customers would not be liable for any stolen funds, or fees or charges incurred as a result of the card cancellations.
Authorities are believed to be investigating the security breach.