Telstra has apologised for a "deeply disturbing" technical issue that prevented Australians from speaking to trained triple zero call takers for more than an hour.
The telco receives all triple zero calls before transferring them to local emergency services but it was unable to do so between 3.30am and 5am AEDT on Friday.
It received 494 calls during that time and was unable to transfer 148 immediately, chief executive Vicki Brady told reporters in Sydney.
She said Telstra was aware of a call to triple zero about a person who suffered a cardiac arrest and died.
"I just first offer my deepest apology to the family of that person and in fact anyone who was impacted in those 90 minutes," she said.
Ms Brady said she had not yet spoken to the family.
"I haven't yet had a chance and (it is) not appropriate yet to reach out to that family but clearly we don't yet understand how much the delay was and how much that impacted,'' she said.
"But obviously it's unacceptable if there was any delay in getting that call through."
Victorian Ambulance Union secretary Danny Hill said that the incident in inner Melbourne was distressing for the family members involved and paramedics who tried to revive the patient.
He said the family struggled to get through to triple zero and later received a call back after their loved one had died.
"The patient sadly couldn't be revived and passed away" Mr Hill told AAP.
"After that, there was a phone call to the person's house asking if they still needed an ambulance, and that was about an hour after the original call.
"So, incredibly distressing for the family at an already difficult time to get that call, but also quite distressing for the person who had to make the follow-up phone call as well."
Paramedics described the situation as "chaos", Mr Hill said, as crews were dispatched to cases without knowing the details of the emergencies.
Ambulance Victoria will review all patients assessed during the outage and follow up on them, spokeswoman Danielle North said.
"We're working to review all cases during this period to identify if there have been any impacts to patient care," she said.
"We know this would have been a distressing experience for people unable to get through to Triple Zero Victoria during this period."
Ms Brady said during the outage Telstra staff took down details from triple zero calls and manually sent information to emergency services by email.
"We followed our backup process exactly as they are designed but it is clear already from our preliminary review that there are improvements that we can make in these processes," she said.
The telco is investigating what caused its servers to not work as intended, which impacted calls in several states.
Communications Minister Michelle Rowland said reports of Australians unable to reach emergency services were deeply concerning.
The Australian Communications and Media Authority was assessing Telstra's compliance with its regulatory obligations, she said.
"The government understands that one of the impacted callers to triple zero has passed away," Ms Rowland said.
"We are deeply saddened and our thoughts are with their family and friends."
Measures to ensure the service's resilience had to be identified, the minister said.
