A 12-hour outage on Wednesday left 10 million individuals and businesses unable to make or receive calls, or complete transactions.
The company said in a statement on Monday the cause was now known and steps had been taken to ensure it won't happen again.
"At around 4.05am Wednesday morning, the Optus network received changes to routing information from an international peering network following a routine software upgrade," the company said.
"These routing information changes propagated through multiple layers in our network and exceeded preset safety levels on key routers which could not handle these.
"This resulted in those routers disconnecting from the Optus IP Core network to protect themselves."
The time taken to restore the system was longer than anticipated as some of the routers needed to be physically rebooted, requiring Optus staff to be deployed across a number of sites across the country.
"The restoration of the network was at all times our priority and we subsequently established the cause working together with our partners," the telco said.
"We have made changes to the network to address this issue so that it cannot occur again."
Optus will co-operate with reviews launched by the government and the Senate.
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