The Telstra outage that left tens of thousands of customers' without NBN and ADSL services last week was caused by a faulty software update, the telecommunications giant has revealed.
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Chief Operations Officer Kate McKenzie said Telstra had run a software update on one of Telstra's domain name servers on Thursday, which caused that server to crash.
The fault was fixed by Friday morning, Ms McKenzie said, but Telstra did not anticipate its flow-on effect to customers' modems.
"A number of the modems sitting on the end of the network didn't behave in the way they were supposed to," she said.
"So since that time we've had to go through pretty much modem by modem, and domain by domain and customer by customer to figure out what their issues were."
About 370,000 Telstra customers - or 10 per cent of all clients - lost internet access in the initial outage, with 33,000 still disconnected on Friday and 15,000 unable to log on over the weekend.
Ms McKenzie said some customers - numbering in the "single-digit thousands" - had still not had their connections restored. Telstra is sending new modems out to those customers.
"We are incredibly apologetic to our customers for the inconvenience that's been caused to them by this recent service interruption," she said.
Ms McKenzie said Telstra continued to have "absolute confidence in our great network."
"We've invested in it substantially and continue to do so. It's one of the most advanced networks in the world."
Asked whether the telco would offer affected customers another "free data day", as it did after previous outages, Ms McKenzie said customers had indicated they weren't interested. Some have been offered a $25 discount from their bills.
Telstra chief executive Andrew Penn had tweeted earlier this week that he was reading all their complaints on social media.