Optus, the country's second-biggest telecoms firm, suffered a 13-hour disruption to emergency call services last year, possibly causing four deaths.
That followed a 2022 cyberattack exposing millions of people's personal details and a 2023 outage, which left millions without phone or internet for a day.
In her opening statement to the Senate, Brady said maintenance work on a network timing and synchronisation equipment triggered a software configuration that reset the device's date to 2006.
This resulted in authentication certificates failing across Telstra's network, disrupting voice and data services, including calls to the Triple Zero emergency line.
The issue happened because an intentional design change to the time-keeping device, meant to fix a prior fault, had not been properly documented, leaving the maintenance team unaware of how the equipment would behave when restarted.
A software update had also not been applied to the device.
"Had that software update been completed or had the design change been properly reviewed and documented post the earlier incident, and reflected in the maintenance procedure, the outage may not have occurred," Brady told the Senate.
"Our investigation will address why that design change was not documented, why the software update was not completed, and what needs to change in our controls so known risks are captured, prioritised and closed before they can affect customers."
Brady joined Telstra in 2016 from Singtel-owned (STEL.SI) Optus.
