Optus' network was set up for failure from the moment it embarked on an upgrade that severed Australians' access to the triple-zero system.

Chief executive Stephen Rue has outlined the causes of the outage on Thursday as the telco's Singapore owner circled the wagons around the embattled Optus boss less than a year into his tenure.

Mr Rue blamed the issue on human error, brushing off concerns parent Singtel had cut spending to Australia's second largest telco.

"Standard processes were not followed," Mr Rue told reporters in Sydney on Wednesday.

"That's not an investment issue, that's people not following process."

"That is a result of human error."

The first step of Optus' firewall upgrade was to divert calls to a separate part of the core network.

But preliminary investigations rev

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