Hundreds of emergency calls failed during Optus outage

"We absolutely believe that the triple-zero system should have worked and it's critical for all Australians that the system can be relied upon," she told a Senate inquiry on Friday.

"(But) we don't manage the triple-zero system, it's a very complex system that involves all the carriers, it involves the device manufacturers."

Asked by committee chair Sarah Hanson-Young if Optus knew why customers were unable to make the calls, Ms Bayer Rosmarin said they had done some inquiries but couldn't investigate themselves because of "complex relationships".

"It's too early to tell where the issue actually occurred," she said.

"The triple zero system is supposed to be able to pick up the traffic when we have an outage like this."

Senator Hanson Young accused Optus of attempting to "share the blame" when they should be accepting responsibility, apologising and accept a penalty.

"It's not anybody else's fault that Optus customers couldn't call triple-zero, surely it's Optus's fault," she said.

The Optus chief also revealed the outage was so unusual that they had not planned for it.

"We didn't have a plan in place for that specific scale of outage," she said.

"It was unexpected - we have high levels of redundancy and it's not something that we expect to happen."

During the outage, millions of Australian individuals and businesses were unable to make calls, access the internet or complete transactions.

In response, the company apologised and customers - including businesses that lost thousands in sales - were offered 200GB of extra data, or free data on weekends if they were on prepaid plans as a "gesture of thanks for their ongoing support and patience".

Ms Bayer Rosmarin opened her address to the inquiry by acknowledging the pain of her customers.

"While I believe wholeheartedly that we did everything we could to provide timely, accurate and credible information (but) I acknowledge that there is always more we could have done," she said.

"Beyond restoring our network, our focus now is restoring trust."

But the wrath of senators quickly began to fall on the Optus boss.

"You provide a service to over 10 million people and not just individuals -  government agencies, emergency services, businesses - and all they got for hours, was a couple of lines that said, 'sorry our services out we're working on it," Senator Hanson-Young said.

"For a communications company, the communication is pretty lousy."

Greens Senator Sarah Hanson-Young says the Optus response was not good enough.

The company initially blamed the outage on a routine software upgrade but their updated submission to the inquiry reveals it was the result of its routers' default settings.

The Cisco routers had automatically self-isolated to protect themselves from an unexpected overload of IP routing information after a software upgrade.

"Although the software upgrade resulted in the change in routing information, it was not the cause of the incident," the submission states.

It took longer than expected to restore the system because some of the routers needed to be physically rebooted in a "brute force resuscitation" which required Optus staff to be deployed to a number of sites across the country.

The outage began at 4.05am and was addressed in a crisis meeting at 7.45am, after which Ms Bayer Rosmarin directly contacted Communications Minister Michelle Rowland.

The outage came just over a year after Optus fell victim to data breach that compromised the information of millions of Australians and caused the Medicare, licence and passport numbers of 10,000 customers to be stolen and leaked online.