The chairman and CEO of embattled telco Optus have been grilled at a Senate inquiry into the catastrophic triple-zero failure that left three people dead.

Human error during a firewall upgrade in September led to an outage in multiple states and territories, which led to customers being unable to call the emergency line.

Speaking at a Senate inquiry today, Optus CEO Stephen Rue admitted the mistakes made by the telco during and immediately after the outage.

"There is no doubt that the timeline shows there were delays, there were mistakes made," he said.

"The tragic deaths of people during this outage stay with us as individuals and as a company as we investigate the incident.

"I fully accept there are aspects to the way events unfolded and how they were communicated over S

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