This is the best summary I could come up with:

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.

"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.

Before Monday's disclosure by Optus, experts had theorised the outage was likely a "regular software upgrade gone wrong".

"The problem is too widespread to be due to a cable break or equipment failure," said Tom Worthington, a senior lecturer in computer science from the Australian National University in Canberra.

The software upgrade theory surmised by telecommunications analysts and experts last Wednesday were put to Optus CEO Kelly Bayer Rosmarin, who rejected those suggestions.

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.

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