A "massive messaging storm" overwhelmed a portion of Telstra's domain name server infrastructure for several hours on Sunday, causing widespread internet problems for mostly east coast customers.
The problems started at around 10.30am AEST and were resolved by about 2.30pm.
Telstra initially said in a statement on Twitter that some of its DNS infrastructure was targeted by a “cyber attack, known as a denial of service (DoS)”.
“We're blocking the malicious traffic attacking some of our services,” it said in a separate statement just after midday AEST.
“We are confident we have blocked all of this malicious traffic and are working to get you back up and running again.”
However, the telco revised its assessment of the root cause after bringing the infrastructure back online.
"The massive messaging storm that presented as a denial-of-service cyber attack has been investigated by our security teams and we now believe that it was not malicious, but a Domain Name Server issue," Telstra said, without elaborating.
Customers that switched their DNS to the likes of Google’s 220.127.116.11 were able to resolve website addresses while Telstra's DNS was down.
This story was updated 2.45pm to revise the stated cause of the outage.