The national Telstra outage last week was triggered by maintenance work that caused the system's clocks to go back two decades, the telco says.Telstra has released its submission to today's Senate inquiry, in which chief executive Vicki Brady and senior executives will be questioned over the outage that disrupted business, transport and access to the Triple Zero (000).It said the outage happened after a team carrying out the maintenance work followed the procedures as documented, but something went wrong."The NTP server in Exhibition St had an underlying software configuration, such that when the device restarted, it reset the date to 2006," Telstra said."We now believe this occurred because of an intentional design change that had previously been made to the equipment to fix an earlier fault."That design change had not been properly documented, which meant the maintenance team was not aware of the way the device 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 documented and reflected in the maintenance procedure, the outage may not have occurred," Telsta said.The telco could be fined millions of dollars over the outage and face hundreds of millions of dollars' worth of compensation claims.Ms Brady made a public apology last week."We take trust in Triple Zero extremely seriously," the submission said."We know and fully accept our responsibility to do everything in our power to make sure calls are answered and transferred immediately, and the outage across 8 and 9 July prevented some of our customers from doing that,."We now understand the likely technical cause of the outage from our initial investigation. The team carrying out the maintenance work followed a defined maintenance procedure, however, when the device was re-powered, a GPS card within it did not operate as expected."We now believe this occurred because of an intentional design change that had previously been made to the equipment to fix an earlier fault had not been properly documented."This meant the maintenance team was not aware of the way the device would behave when restarted."Telstra said had the 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"."That is clearly unacceptable," the submission said."If maintenance work can trigger this kind of outage, it suggests our controls were not good enough."We are accountable for that, and 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."We are thoroughly reviewing the incident to learn how we can do better."