Optus has apologised for a second triple-zero outage in as many weeks as failures to re-route calls through other networks are back in the spotlight.

The latest blackout - dubbed "unacceptable" by NSW Premier Chris Minns - is linked to a mobile phone tower in NSW that affected 4500 users in the Dapto area on Sunday between 3am and 12.20pm.

A review by the telco identified nine failed calls to triple zero, but Optus says those affected "are OK" and the issue has been rectified.

The NSW premier lambasted the failure in the Illawarra region as "clearly unacceptable."

"NSW deserves full and transparent information from Optus about what went wrong yesterday, including when emergency services and the Telco Authority were notified," Mr Minns said on Monday, referring to the state's critical c

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