Optus has revealed the cause of the nationwide outage last week which lasted for most of a day and saw mobile, network and landline connectivity obstructed including Triple Zero calls.
The telecommunications company released a statement saying its network received changes to routing information from an international peering network.
The statement claimed the incident occurred during a routine software upgrade.
The restoration required a large-scale effort of the team and in some cases required Optus to reconnect or reboot routers physically.
Optus says it has since made changes to its network to address the issue to avoid it happening again.