National Australia Bank (NAB) says problems with its Internet banking services this week have been caused by a malicious distributed denial-of-service attack.
In a statement NAB says some customers have experienced slow or intermittent access to its Internet banking system because of "a deliberate attempt by external parties to slow or stop NAB's online services".
"They did this by jamming NAB's connection to the Internet with hits making it more difficult for legitimate users to gain access," says NAB's chief information officer Michelle Tredenick.
Tredenick says customer information has not been at risk as a result of this issue and the bank's security applications have protected internal systems.
Following the attack, the bank has renewed its warning of hoax phishing e-mails that are designed to fraudulently obtain personal information from customers.