Optus executives have copped a parliamentary bashing for their response to a triple-zero outage linked to the deaths of three people.

Chief executive Stephen Rue was in the firing line from coalition and crossbench senators for taking more than six hours to tell the communications minister and industry regulator about a massive increase in the scale of the September outage.

It prevented more than 600 triple-zero calls from connecting when Optus originally suggested the number involved was just a handful.

Liberal senator Sarah Henderson told a senate inquiry on Monday that the deaths were preventable, after Rue said the company regretted not moving sooner on reforms that would have detected the outage earlier and better protected customers.

“Optus never detected the outage, which I’m shocked about … for hours and hours and hours, Optus did not know what was going on,” she said.

Greens senator Sarah Hanson-Young questioned why senior Optus management were not told for hours that multiple deaths had been linked to the outage.

She also queried why there was a six-hour delay in notifying the Australian Communications and Media Authority or Communications Minister Anika Wells when Rue was personally informed about the seriousness of the issues.

“What on earth were you doing between 8am and 2pm?” Hanson-Young said.

“You were too busy putting your ducks in order, telling your board what was going on, contacting your executives … meanwhile, the federal government, the regulator and the minister, were left in the dark.”

A technical problem with Optus’s triple-zero calls affected hundreds of customers.

Rue defended the delay, arguing Optus was conducting welfare checks and wanted to provide government officials with accurate data.

“The judgment I made was it was best to get the information accurately together and then inform the regulator, the department and the minister’s office.”

He told the senate inquiry Optus had “fulfilled the regulatory requirement, which was to declare that there was a significant network outage”.

The triple-zero outage was caused by human error during a routine firewall upgrade, meaning triple-zero calls were not diverted to another network, officials said.

A timeline presented to the hearing showed frontline staff learned of two deaths around 9pm on the Thursday evening.

Emails sent to senior executives were not read until the following morning, after which Rue was alerted. Then it was around another six hours before the regulator, the Australian Communications and Media Authority (ACMA) was told.

ACMA 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.

Optus announced on Monday that 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 following the incident.

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 Rue said he had asked relevant authorities to see if they wanted to speak with him.

Henderson 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 both 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.

–AAP