Triple Zero Victoria – whose operators then ask for more detailed information – said it was told the Telstra triple-zero service disruption affected multiple states.

“Throughout the disruption, Telstra provided Triple Zero Victoria with the details of callers who were responded to as a matter of urgency,” a spokesperson said in a statement.

Danny Hill, the Victorian Ambulance Union secretary, told the ABC the outage at the first point in the triple-zero call process caused “complete chaos” for paramedics. He later confirmed the cardiac arrest call-out during the outage was for a man in Victoria.

“It’s very difficult to determine if the situation would’ve been different for the gentleman who has passed away, without a delay,” Hill told 3AW.

“What we do know is that, when there is a delay, you are really robbing them of their best chance to survive a cardiac arrest.”

Hill said triple-zero operators called back cases they thought could have been delayed. An hour after the Victorian man died, they called back asking if an ambulance was still needed, the union boss said.

“Obviously, it’s distressing for everyone,” he said.

Communication Workers Union secretary Sue Riley, who represents the triple-zero operators Telstra forwards calls to, told the ABC the telco’s dispatchers resorted to sending emails en masse with phone numbers to emergency services about 4.15am.

Telstra’s dispatchers, who aren’t trained like Victoria’s triple-zero operators, captured as much information about each emergency as they could, but sometimes didn’t record basic details like an address, Riley said.

“I think everyone did the best they could in the circumstances,” Riley said, later adding: “[Telstra] are answerable to the public about this.”

During the 90-minute outage, Telstra’s chief executive said 494 triple-zero calls couldn’t be automatically forwarded as usual. Of those, 346 were transferred using a backup method, and 148 had their details emailed to emergency services for a call-back.

“I want to reinforce that triple-zero has our highest priority. We absolutely understand the critical role we play in this vital service to the Australian public,” Brady said.

“As I said, the investigation is under way and my commitment is that it is swift, thorough and forensic, and that we will share findings as we complete that investigation.”

Federal Communications Minister Michelle Rowland said the government was deeply saddened by the fatality during the outage.

In a statement, Rowland said she had sought information from Telstra about the extent of the disruption, and committed to a reliable emergency-call service.

“Whilst no service is 100 per cent immune from temporary disruption, measures must be identified to improve the resilience of the service and its backup processes,” she said.