The 14-hour outage during a network upgrade in September hit South Australia, Western Australia, the Northern Territory and parts of far west NSW.
Of 605 emergency calls in that time, 455 failed to connect.
An independent review, released by Optus on Thursday, found there were at least 10 mistakes by the company and its contractor Nokia during the upgrade.
It took Optus and Nokia 13 hours to understand the problem, with an Optus call centre not escalating early concerns from five people who said their triple-zero calls weren't working.
Optus and Nokia also "astoundingly" attributed early alerts about the problem at a networks operations centre to the fact upgrades were happening, the report by experienced executive Kerry Schott said.
Once Optus was aware of the issue it was fixed within minutes.
"The real tragedy here was the duration of the triple zero call outage and the failure of Optus and its contractors to deal immediately with the warning signals they received," Dr Schott said.
The origin of the outage was a failure to divert voice calls, including emergency calls, before closing a gateway at an exchange station in South Australia.
The first of the 10 mistakes was incorrect instructions being sent to Nokia about the upgrade process, the report said.
The mistakes could only be explained by a lack of care and lack of adherence to procedure, Dr Schott said.
Optus has pledged to swiftly implement all 21 recommendations of the report, including for better processes around upgrades and addressing the siloed nature of work at the company.
The telco's chairman, John Arthur, said the failures were unacceptable and the board was taking further action against individuals, including financial penalties and potentially sackings.
"We are deeply sorry for the 18 September outage that impacted triple-zero services and the consequences for any Australians who were trying to reach emergency services," he said.
Dr Schott said it was puzzling only 66 callers were able to "camp-on" - a process where emergency calls are redirected through another network.
Some devices took 40-60 seconds to be redirected to another network.
"In an emergency, people are unlikely to hang on for this length of time, especially when the only response they are getting is silence on the line," Dr Schott said.
"This delay is something that customers at all telecommunications companies should be informed about."
Standards and requirements for devices and carriers to ensure emergency calls are successful may not have kept up with technological changes, she said.
The Greens have called for a government review into camp-on provisions and a review of Optus' licence and licence conditions.
The federal government has helped establish the National Telecom Resilience Centre, which tests emergency calling scenarios including camp-on.
The outage was unacceptable and the nation's independent communications regulator was continuing its investigation, a spokesman for Communications Minister Anika Wells said.
"Access to triple zero is one of the most critical requirements for mobile carriers and work is continuing to improve the system's resilience," the spokesman said.
