This was billed as Optus coming clean on the cause:

Optus: At around 4.05am Wednesday morning, the Optus network received changes to routing information from an international peering network following a routine software upgrade.

What's an international peering network? Why did it receive changes that would close it down?

Prof Mark Gregory, RMIT (of Fact-Check infamy): (I)t appears that a routine software upgrade to one or more key routers was the cause of the outage. .. (W)e now know that the Optus outage was not hardware failure and not related to a cyberattack.

How do we know that for sure? A normal way to commit cyber-attack is to infiltrate the update. What's the other explanation, again? A routine software upgrade crashes the entire system? I'm not sure which is more disturbing.

Optus: 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. This is why restoration was progressive over the afternoon.