Police are working to determine whether a fourth person died after being left unable to contact triple zero due to an Optus outage.

The deaths of two adults and an infant after efforts to dial through to the emergency number failed during Thursday's 10-hour outage have already been confirmed.

An eight-week-old baby and 68-year-old woman died in Adelaide, while the third fatality linked to the incident, which involved up to 600 attempted triple-zero calls, was that of a 74-year-old from Perth.

WA Premier Roger Cook says police are also investigating the circumstances surrounding a fourth fatality.

"Optus must be held accountable for this incredibly tragic event and they must provide answers," he told reporters on Saturday.

A customer alerted Optus to the outage after the telco failed t

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