Up to 600 households in South Australia, Western Australia and the Northern Territory were impacted by the triple-zero outage during a network upgrade on Thursday, Optus chief executive Stephen Rue said.
"I have been advised that in the process of conducting welfare checks, three of the triple-zero calls involved households where a person tragically passed away," Mr Rue told reporters in Sydney on Friday.
"Please know that these welfare checks are ongoing."Â
He said the technical problem on the network had been resolved.
The boss of Australia's second-largest telco provider offered "a sincere apology to all customers who could not connect to emergency services when they needed them most".
"I offer my most sincere and heartfelt condolences to the family and friends of the people who passed away - I am so sorry," he said.
"What has happened is completely unacceptable."
The chief executive said the duration of the outage was not yet known and further details would be made public when an internal investigation was completed.
Communications Minister Anika Wells said Optus's conduct was "incredibly serious and completely unacceptable" and would be investigated by federal authorities.
"The impact of this failure has had tragic consequences and my personal thoughts are with those who have lost a loved one," she said in a statement.
"While details are still emerging, no triple-zero outage is acceptable. Optus and all telecommunication providers have obligations to ensure they carry emergency services calls."
The glitch comes after Optus paid more than $12 million in penalties in 2024 for breaching emergency call rules during a nationwide network outage a year earlier that caused significant disruption.
In the 2023 incident, Optus failed to provide emergency call access to 2145 people and subsequently did not conduct welfare checks on 369 people who tried to call triple zero, the communications watchdog found.
Mr Rue took over as the company's chief executive in 2024 from Kelly Bayer Rosmarin, who resigned after the nationwide outage.
Australian Communications Consumer Action Network, a consumer advocacy organisation for communications, said Optus's latest failure came despite a thorough federal review following the previous high-profile outage.
The network's chief executive Carol Bennett told ABC News telcos were "aware that this has a really big impact" on communities and their trust in telcos.
She hoped Optus's probe would urgently look at the technical issues that sparked the outage and the timeliness of its response.
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