ANZ, CommBank and Westpac all faced issues with their online banking services on Thursday thanks to a problem with a DDoS mitigation tool run by cloud services provider Akamai.
A host of other financial sites, including the Reserve Bank of Australia, were also briefly affected by the blackout on Thursday. The Australian postal service and news sites were also hit.
The cause is understood to be a technical issue with Akamai's Prolexic platform, which is supposed to protect against DDoS attacks.
The outages come just days after a problem with Akamai rival Fastly knocked out thousands of government and news sites.