The Commonwealth Bank is yet to confirm the source of a partly malicious traffic payload that has taken its internet banking site offline several times since yesterday.
A bank spokeswoman said it was "critically aware" of the issues and was "working as a matter of urgency" to fix NetBank.
She said that there was no security breach and that no customer information or funds were compromised.
The outage has the hallmarks of a denial-of-service (DoS) attack: "We are experiencing exceptionally high volumes of traffic, some of which appears to be malicious," she said.
"The source and type of this is yet to be confirmed".
Commonwealth said there were "planned, intermittent periods" where NetBank was down due to work on its systems being prioritised over customers. Outages could continue until the source of the traffic is found.
It said the outages ensured privacy of customer data.
"Customer information and privacy must be secured at all times. If that is ever under threat we must forego convenience for a short time whilst services are assured," the spokeswoman said.
Customers requiring assistance, who have questions or concerns about NetBank should call its helpdesk on 132 221.