A fourth person died after being unable to contact the triple zero emergency line during a 10-hour Optus outage, it has been revealed.

The telco has confirmed a second death in Western Australia and says police informed them the person had likely attempted to call triple zero for help.

"I am deeply saddened by this further news and extend my heartfelt condolences to the person's family and friends," Optus CEO Stephen Rue said in a statement announcing the death on Saturday afternoon.

An eight-week-old baby and 68-year-old woman also died in Adelaide during the outage, as did a 74-year-old man in Perth. Some 600 triple-zero calls were attempted during the interruption.

WA Premier Roger Cook said "Optus must be held accountable for this incredibly tragic event and they must provide answe

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