Staff at the embattled telco learned before 9pm on September 18 multiple fatalities had been tied to the catastrophic outage, but they waited until the following afternoon to let the industry regulator and minister know.
Under-fire executives from Optus copped a parliamentary bashing on Monday for their response to the triple-zero failure and three linked deaths.
More than 600 triple-zero calls could not be connected, but the telco initially suggested to authorities the number involved was just a handful.
Optus chief executive Stephen Rue said the company wanted to complete welfare checks on those affected by the outage before presenting authorities accurate data.
"We fulfilled the regulatory requirement, which was to declare that there was a significant network outage," he told a senate inquiry.
"I was gathering the information to be able to give accurate information which was outside of the regulatory obligation."
The behaviour did not make sense, Greens senator Sarah Hanson-Young said.
"I'm struggling to understand how alarm bells weren't rung that the information you had given to the regulator was false," she said.
"You didn't give any further information or update, even when you knew on (September 18) it was worse … if this is how you run your organisation, no wonder it's in a mess."
Australian Communications and Media Authority chair Nerida O'Loughlin admitted she was surprised Optus took so long to update the regulator.
"Our expectation would be the telco, in that circumstance, would have kept us up to date on significant changes to the information they have provided … that was not the case," she said.
The telco announced 300 people would be added to its Australian call centres with a focus on the emergency network, while safeguards surrounding triple-zero calls would be ramped up.
Mr Rue, who has faced calls for his sacking after the outage, said the introduction of new executives could hamper the work Optus had done to increase triple-zero network protections.
Company chair John Arthur backed the chief executive to keep his job.
Both declined to provide information about what contact Optus had with the families of those who died during the outage, although Mr Rue said he had asked relevant authorities to see if they wanted to speak with him.
The triple-zero outage was caused by human error during a routine firewall upgrade, meaning triple-zero calls were not diverted to another network, company officials said.
Liberal senator Sarah Henderson said the deaths were preventable after Mr Rue said the company regretted not moving sooner on reforms that would have detected the outage earlier and better protected customers.
She said Optus must pay compensation to the affected families.
"They provide a service, that service failed," she said.
"Optus has got huge liability and we demand answers as to what they are going to do for these families."
The probe was set up to better understand what caused the September outage, which stopped hundreds of Australians from making triple-zero calls.
It will also examine the effectiveness of emergency arrangements designed to shift customers to another network if their telco has an outage.
The communications watchdog and Optus are running their own investigations into the outage.
Rules that took effect on Saturday require telcos to report outages to the communications watchdog and emergency services in real time.
