The September 18 outage, which lasted almost 14 hours, affected hundreds of triple-zero calls in four states and territories and has been linked to two deaths due to emergency calls failing.
Offshore call centre staff repeatedly failed to escalate warnings from customers about the triple-zero outage, which was caused in part by errors in the telco's outsourced network management team.
Hours later, a customer contacted Optus directly about the outage which is when the company became aware of the severity of the failure, a parliamentary inquiry into the incident was told on Thursday.
Mr Rue confirmed there would be up to 300 job cuts at Optus as it tried to reset following the outage.
He agreed Optus had a "culture of carelessness".
"The transformation program ... is always going to take time, I'm afraid," Mr Rue told the inquiry.
"But a key component of that is culture. It's culture and risk."
He said several other changes had been made since the fatal blunder, including onboarding new onshore call centre roles, strengthening escalation processes and improving real-time visibility of triple-zero performance.
Kerry Schott, who completed an independent review into the deadly incident, said when the outage was referred to the call centre hours later it was after the knowledge that two fatalities occurred.
"At that time, they were extraordinarily distressed," she said.
"Senior people at the call centre actually seemed to spend an hour or more just calming everyone down.
"The management of the call centre at that time was not adequate, and has since been changed, and was changed very quickly after this event."
Dr Schott said it was astounding alerts were missed and blamed it on a "culture of lack of care".
"It's a culture that I can understand how it crept up and happened, but it is unforgivable and it must be fixed," she said.
In her review, Dr Schott handed down 21 recommendations and described the failings as inexcusable.
Optus has agreed to implement all recommendations.
Dr Schott said a 40-page crisis management document Optus used was too dense for employees to easily follow.
She recommended the telco condense the document into a checklist, so staff had clear instructions in an emergency.
Optus extended its "deepest sympathies" to the families and friends of the people who died.
"It weighs heavily on me and our board and on the people of Optus," Mr Rue said.
"We recognise the seriousness of what happened and the responsibility we bear."
The federal government will review the laws overseeing the emergency services network.
NSW Ambulance Assistant Commissioner Brent Armitage said the organisation would contribute to the possible drafting of a national framework that identified where there were issues reaching the triple-zero network.
"I think consistency across a large jurisdiction, which includes multiple emergency service organisations across different states and territories, would be the best way to go," he said.
"It would be good to ensure that there's a consistent national standard that can be applied."
Greens communications spokeswoman Sarah Hanson-Young said the grieving families deserved significant compensation.
"This corporation has been making profits off the back of their customers and putting their customers safety last," she told reporters.
"It's time for that change. It's also why Optus should be getting the maximum fines available."
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