Three people died during an Optus triple zero failure across South Australia, Western Australia and the Northern Territory that was the result of a network update issue.
Optus chief executive Stephen Rue said a technical failure affected about 600 triple zero calls on the Optus network.
Rue said welfare checks were then conducted at all affected households after the issue was fixed.
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From those welfare checks it was established that three of the calls came from households where people passed away.
Two of the deaths took place in SA and another in WA.
"Our investigation is ongoing but at this stage I can confirm that approximately 600 customers were potentially impacted, of which a portion of calls did not go through," he said.
He apologised to the families of those who passed away.
"I am so sorry for your loss What has happened is completely unacceptable. We have let you down. You have my assurance that we are conducting a thorough investigation.
"This should not have happened."
It's not the first time Optus has had a major outage.
Optus was last year ordered to pay more than $12 million for breaches of emergency call rules in November 2023 following an investigation by the Australian Communications and Media Authority (ACMA).
The ACMA found Optus failed to provide access to the emergency call service for 2145 people during the course of the 14-hour outage on November 8, 2023.
It said Optus then failed to conduct 369 welfare checks on people who had tried to make an emergency call during the outage.
At the time the fine was issued, ACMA Chair Nerida O'Loughlin said significant penalty reflected the critical nature of the breaches.
"Triple zero availability is the most fundamental service telcos must provide to the public. When an emergency call fails to connect there can be devastating consequences for public health and safety," she said.
