Telstra took full accountability and said its investigation will determine why the design change wasn’t documented, why the update wasn’t applied, and what changes are needed to prevent repeats.

At the outage’s peak, about 45 percent of all calls and data sessions on Telstra’s mobile network were affected, surpassing earlier estimates.

Maintenance staff were unaware of a design change meant to fix an earlier fault because it wasn’t properly documented, and a software update for the device had not been applied.

Telstra attributed the nationwide triple-0 outage to multiple failures, including an undocumented design change and a missing software update on its timekeeping system, which caused time data to default to 2006 and cascaded into authentication certificates across the network.

The incorrect date from the timekeeping system led to invalid certificates on other servers, disrupting voice and data across Telstra’s mobile network, including emergency calls to triple-0.

One Melbourne-based NTP server reset to 2006 during maintenance due to an underlying software configuration, causing a ripple of invalid authentication across the network.

Executives emphasized the need for stronger controls to capture, prioritize, and close known risks before they affect customers.

During the outage, roughly 58,800 triple-zero calls connected while about 600 encountered errors; the triple-zero platform itself remained unaffected since it doesn’t rely on the affected NTP servers.

The inquiry, led by Greens senator Sarah Hanson-Young, is also examining last year’s Optus outage, with Brady and other Telstra executives set to testify to address the incident and broader system vulnerabilities.

Brady walked through a timeline showing a Network Time Protocol server restart with an incorrect date that triggered the fault early in the morning, with call and data failures detected about an hour later.

Telstra said the GPS issue traced to that undocumented design change and the missing software update on the device.

Telstra chief executive Vicki Brady apologized in a Senate inquiry, pledging transparency and acknowledging that the outage let Australians down and eroded trust in the emergency calling system.