Under-fire Optus CEO Stephen Rue has admitted the telco still has no idea why customers were unable to make triple zero calls during a 13-hour period - as it's revealed a fourth person may have died as a result of the outage.

A 74-year-old man from Western Australia and a 68-year-old woman and an eight-week-old baby from South Australia all died after their calls to emergency services were interrupted during the network upgrade.

Western Australian Premier Roger Cook told a press conference on Saturday, that a fourth person, the second in his state, may have also died as a result of the outage.

'I am further advised that as a result of WA Police undertaking welfare checks, that there is potentially another case of a Western Australian that has been attempting to call triple zero and passed away, he said.

'We are awaiting confirmation of this latest incident. This is a dreadful tragedy and it's so disappointing that it has occurred and it is so disappointing about the way that the situation has been revealed, both to government, to the authorities and to the public.'

It comes as Mr Rue was grilled by the media during his own press conference.

The chief executive was asked why Optus customers were able to make regular calls to family and friends but not to emergency services.

He said an internal investigation was still probing what triggered the technical error and why Optus had received no internal notification that triple-zero calls were failing.

Optus CEO Stephen Rue (pictured) said the telco still didn't know why emergency calls failed during a 13-hour period while the telco was undergoing a network upgrade

'It was during an upgrade to a firewall upgrade,' Mr Rue told reporters.

'When we have the exact information, I will share that with you. That is part of our internal investigation at the moment.'

Mr Rue said during a system upgrade at 12.30am on Thursday initial testing did not indicate there were any issues, and normal calls were connecting.

He said there were two complaints made to the Telecommunications Industry Ombudsman about triple zero calls not going through.

The first complaint came about 9am on Thursday, but it did not receive the relevant escalation at the time and so Optus did not get that report.

Optus only found out about the triple zero failure when they were contacted directly by a customer at about 1.30pm on Thursday - a full 13 hours after the outage.

The major outage affected calls to emergency services in South Australia, the Northern Territory and Western Australia.

Mr Rue said there was a technical failure in the system but there were 'no alarms to alert us' that some emergency calls were not making it through to emergency services.

Optus wasn't aware that triple zero calls were failing for 13 hours (pictured, an Optus store)

'The loss of the lives of three people, two in South Australia and one in Western Australia is absolutely tragic,' the CEO said on Saturday.

'I would like to extend my deepest condolences to their families and friends, and I promise that we will fully cooperate with any and all investigations in relation to this.

'In terms of our own investigation into the technical failures, I can confirm that this is ongoing and I will be fully transparent as we establish the facts of the matter.'

Mr Rue was asked if Australians could still trust Optus, after it suffered the biggest mobile network outage in Australia's history in November 2023.

'I will always be transparent with them... which is why I'm standing here today, which is why I'll stand here again tomorrow,' he replied.

The Optus boss referred multiple questions to an upcoming external investigation, saying the results will give him a better picture of how events unfolded.

When asked if the outage was the direct reason three people died, Mr Rue said: 'It's not for me to comment specifically on the families. And again, when we have all the information, we will share that with you'.

Mr Rue said while he had not personally reached out to the families of the three people who died, he would do so at 'the right time'.

His press conference came after Minister for Communications Anika Wells (pictured) said she was staggered the triple zero outage occurred

The 'huge' consequences for Optus after three people including a BABY died during triple-zero outage

His press conference came after Minister for Communications Anika Wells said she was staggered the triple zero outage occurred.

'There was a full review into what happened with the Optus outage in 2023. Those recommendations were accepted by government,' she said.

'Many of the things that happened in this outage are failures to implement some of those recommendations, including alerting the public or emergency services authorities. And that is what I find to be particularly disappointing.'

South Australian Premier Peter Malinauskas has been scathing of Optus for its handling of the situation.

'I have not witnessed such incompetence from an Australian corporation in respect to communications worse than this,' Malinauskas said.

'I cannot believe that anyone in the senior levels of Optus thought they should craft a media statement and conduct a press conference before advising the South Australian government that they had ascertained two deaths had occurred.

'I think quite frankly that is reprehensible conduct on behalf of Optus.'

Optus CEO admits telco STILL doesn't know why triple-zero calls failed