Optus sent emails notifying the federal communications department about a deadly triple-zero outage to the wrong email address, leaving authorities in the dark for more than a day.

The telco's startling mistake was revealed in a parliamentary hearing on Wednesday, along with the admission it underplayed the severity of the issue.

The Singaporean-owned telco sent two emails to the department on September 18, the same day a routine firewall upgrade knocked out emergency calls for more than 12 hours.

But the emails were sent to a redundant address, meaning the department didn't learn about the outage until a phone call from a regulator at 3.30pm the next day, more than 36 hours after the outage began.

"That communication ... was sent to the wrong address, which we have told industry a num

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