NetBank customers must answer two identification questions before conducting some online transactions, such as first-time third-party payments, or international money orders.
"All NetBank customers nominate personalised security questions when registering for NetBank, so they do not need any additional preparation to take advantage of these features," said Commonwealth Bank executive general manager, business and technology services, Quentin Boyes.
The security upgrade is part of a continuous review of security, according to the bank.
"NetBank now uses your Personalised Identification Questions to provide extra protection for your funds and important personal details," the NetBank service tells users.
Also announced were more e-mail alerts to confirm with customers when they have changed their personal details, accounts or billers.
The bank also said it was "examining" two-factor authentication for NetBank in 2006. The Australian Bankers Association has previously flagged that most banks will adopt an extra layer of security for online transactions in the next 12 months.
NetBank claims 2.3 million customers.