In a submission to a parliamentary inquiry into the failure, the telco said a GPS card within a device that was repaired during scheduled maintenance did not operate as expected once power was restored to the network.
"We have let our customers and Australians down, and for that we are deeply sorry," Telstra said in the statement.
The faulty GPS card occurred because of an "intentional design change" that had previously been made to the equipment to fix an earlier fault.
The change had not been properly documented, meaning the maintenance team was not aware the issue would occur when the device was restarted.
A software update, which could have averted the outage, had also not been applied to the device.
"That is clearly unacceptable. If maintenance work can trigger this kind of outage, it suggests our controls were not good enough," Telstra said.
"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."
The submission was released hours before Telstra's bosses were set to be grilled over the outage on Friday.
Greens Senator Sarah Hanson-Young said telcos had been putting profits ahead of people and there needed to be stronger rules to protect consumers than a slap on the wrist.
"Millions and millions of Australians have been impacted by Telstra's bad performance, from Telstra's incompetence, and we'll be asking questions of the CEO and the executives about that today," she told reporters.
The telco has been in damage control since the outage last week, which left thousands of Australians unable to make calls or access data.
An EFTPOS payment system used by about 80,000 businesses confirmed it had been impacted, with a number of customers taken offline and unable to process payments.
Train services had to be cancelled after severe network disruptions in both Victoria and NSW, triggering strong criticism from the federal government.
Telstra is accepting compensation claims from affected customers and small businesses who can provide evidence to support their case.
Telstra undertook more than 630 welfare checks on customers that were prevented from having triple-zero calls connected due to the outage.
Telstra chief Vicki Brady has apologised for the failure.
"Last week we let you down, and I am sorry for that," she said in a statement earlier this week.
"When something goes wrong, we're committed to taking accountability, giving you clear information, and fixing issues as quickly as possible.
"We understand what caused the issue and will complete a full investigation and take what we learn to make our network stronger. You have my commitment on that."
Representatives from the Australian Communications and Media Authority and the communications department will also give evidence at the inquiry.
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