Photo: AFP
Optus says "changes to routing information" after a "routine software upgrade" was behind last week's nationwide outage, affecting 10.2 million Australians and impacting 400,000 businesses.
In a statement released on Monday afternoon, Optus says its network was affected by "changes to routing information from an international peering network" around 4:05am AEDT last Wednesday, "following a routine software upgrade".
"These routing information changes propagated through multiple layers in our network and exceeded preset safety levels on key routers which could not handle these," the company said.
"This resulted in those routers disconnecting from the Optus IP Core network to protect themselves."
The scale of the outage meant Optus technicians had to physically reconnect or reboot the system, the telco said, and also meant the investigation into the cause "took longer than we would have liked".
"The restoration required a large-scale effort of the team and in some cases required Optus to reconnect or reboot routers physically, requiring the dispatch of people across a number of sites in Australia," an Optus spokesperson said.
"This is why restoration was progressive over the afternoon.
"Given the widespread impact of the outage, investigations into the issue took longer than we would have liked as we examined several different paths to restoration.
"The restoration of the network was at all times our priority and we subsequently established the cause working together with our partners."
Optus says it has since made changes to its network to address the issue so it does not occur again, and will "continue to invest" to improve its network's resiliency and services.
It comes after Optus made available an extra 200GB of data to customers from Monday as compensation for last Wednesday's outage.
Optus is facing a number of inquiries and investigations as a result of the outage, including a Senate inquiry that will hold its first public hearings on Friday.
Optus CEO Kelly Bayer Rosmarin is currently the only witness to confirm her attendance.
The telco said in a statement that it supports and will "fully cooperate" with the reviews being done by the government and the Senate.
The reason for the outage follows the federal government announcing earlier on Monday that it would require telecommunications companies in Australia to report their cybersecurity measures to avoid a repeat of Optus' cyber hack last year.
Under the laws, telecommunications companies would be classified as "critical infrastructure" that would require their company boards to report to the government on their cybersecurity strategies in the same way energy companies, hospitals and ports do.
- This story was first published by the ABC.