A fourth person has died after being unable to call triple-zero during an Optus outage. Photo: Bianca De Marchi/AAP PHOTOS

Optus is pledging to fully investigate a 10-hour triple-zero outage, accepting responsibility for a number of breakdowns during which at least four people died.

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The telco's chief executive says an independent investigation will be conducted after admitting the outage was caused by a botched firewall update to the system.

Stephen Rue admitted Optus had been contacted by the telecommunications industry ombudsman about two complaints of people trying to inform the

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