The telco has confirmed a second death in Western Australia and says police informed them the person had likely attempted to call triple zero for help.
"I am deeply saddened by this further news and extend my heartfelt condolences to the person's family and friends," Optus CEO Stephen Rue said in a statement announcing the death on Saturday afternoon.
An eight-week-old baby and 68-year-old woman also died in Adelaide during the outage, as did a 74-year-old man in Perth. Some 600 triple-zero calls were attempted during the interruption.
WA Premier Roger Cook said "Optus must be held accountable for this incredibly tragic event and they must provide answers".
A customer alerted Optus to the outage after the telco failed to detect a system failure that blocked the triple-zero calls.
Mr Rue admitted the outage was caused by a botched firewall upgrade.
He said emergency call access was restored about 1.50 pm on Thursday.
Optus was also notified shortly after by South Australian Police.
"There was a technical failure in the system and further there were no alarms to alert us that some emergency calls were not making it through to emergency services," Mr Rue told reporters in Sydney on Saturday.
Welfare checks were conducted into Thursday evening and Friday, with delays in the timing "due to the complexity of pulling records from the network", he said.
The deaths were not confirmed until Friday, he said while repeatedly defending the decision not to alert the public until a snap press conference that evening as Optus ascertained the facts.
"We take full accountability for the technical failure and that we were unaware of this for ... an unacceptable gap in time," he said.
Early reviews suggested two complaints referred by the telecommunications industry ombudsman from two people who contacted Optus on Thursday morning weren't handled "as would be expected", Mr Rue said.
"This information was not surfaced with the relevant escalation at that time."
He confirmed an independent investigation would be launched.
Federal Communications Minister Anika Wells said she found it "particularly disappointing" the problems should have been addressed following recommendations from a review into a similar outage.
More than 10 million Optus customers and businesses were disconnected for more than 16 hours in November 2023.
People could not call triple zero on landlines, although it was still possible to do so on a mobile and the telco was fined more than $12 million for breaching emergency call rules during the nationwide outage.
"I find it hard to put into words how this could have possibly been allowed to happen again," Ms Wells told reporters in Brisbane.
The outage would be thoroughly investigated, she said.
SA police worked through Friday night conducting welfare checks on 150 people whose triple-zero calls failed, Premier Peter Malinauskas said on Saturday.
Optus had a duty to inform authorities as quickly as possible "and the fact that didn't occur until after a press conference on Friday beggars belief", he said.
"They've got to make sure they're letting our emergency services know ... all the information the moment they have it, before they think about crafting a media statement."
The premier was also scathing of Optus for only sharing the suburbs where the deaths had occurred with police, and said it took a call for him to the CEO to rectify this.
It was "somewhat extraordinary" police struggled to get information from Optus in light of what had happened, Mr Malinauskas said.
"The lack of information flow from Optus to the South Australian government's appropriate authorities is somewhat bewildering and it raises a lot of questions."
It was not yet known if the failed triple-zero calls had a direct impact on the emergencies in which people had died.
10°C