Optus has apologised for a second triple-zero outage in as many weeks, as local executives prepare for showdown talks with the embattled telco’s owners.

The latest blackout – blasted as “unacceptable” by NSW Premier Chris Minns – is linked to a mobile phone tower in NSW and affected 4500 users in the Dapto area between 3am and 12.20pm on Sunday.

A review by the telco identified nine failed calls to triple zero. The issue has been rectified, although the cause is not yet known.

“Optus continues to investigate the cause of an issue involving a mobile phone tower site in the Dapto area in NSW. The issue has been restored,” Optus said.

One caller required an ambulance and used another phone to contact emergency services.

Another was trying to call emergency services but couldn’t get through and had made accidental calls.

“NSW deserves full and transparent information from Optus about what went wrong yesterday, including when emergency services and the Telco Authority were notified,” Minns said on Monday, referring to NSW’s critical communications agency.

NSW Police confirmed they received a request from Optus on Sunday morning to conduct welfare checks on several callers in the Dapto area who had tried to contact triple zero and been unable to connect.

Optus has also reported one of the Dapto callers required an ambulance and used another phone to contact emergency services.

Another unable to reach emergency services have confirmed they “are OK”. Two calls were accidental and three were test calls.

Four welfare checks were referred to police but none required an emergency services response. Dapto is a southern suburb of Wollongong.

Federal Treasurer Jim Chalmers said the government was holding the telco to account with a thorough investigation under way by the Australian Communications and Media Authority.

“This can’t happen again. This is an absolutely shocking failure from Optus,” he said on Monday.

The outage has also raised questions about the effectiveness of the camp-on mechanism. It means if a triple-zero call fails to connect from a phone user’s network, it is automatically is routed through another provider.

RMIT engineering academic Mark Gregory slammed Optus and other telcos for not investing necessary resources to deal with outages.

“They don’t want people to really see how bad their networks actually are,” he said.

“The networks are less reliable than they should be and that is because of the lack of investment in the networks and the systems that run those networks.”

Gregory said the technical details of what happened in Dapto must be shared with the public, suggesting other Telstra or TPG towers were not in customers’ range.

He said the onus was also on the federal government to empower the Australian Competition and Consumer Commission and Telecommunications Industry Ombudsman to enforce regulations.

“The telcos have invested heavily in lobbying, and the moment the government is listening to the telcos and not to the regulators,” he said.

Asked about domestic roaming and the camp-on solution, ACCC chair Gina Gottlieb said on Monday the watchdog had recommended in a 2023 report switching calls between carriers in emergencies.

At the time, carriers noted congestion on the surviving network and technical parameters were key issues for temporary domestic roaming.

The Dapto triple-zero failure is the latest in a hat-trick of outages for the besieged Optus.

Nearly two years ago, it paid a $12 million fine after a technical issue left more than 2000 people unable to ring the emergency number.

On September 18, a more wide-ranging communications outage was linked to the deaths of three Australians.

Optus chief executive Stephen Rue has blamed human error for that fault, which was triggered by a scheduled firewall upgrade in South Australia.

Normal calls were largely unaffected but the outage blocked about 600 triple-zero calls from connecting to emergency services in South Australia, Western Australia, the Northern Territory and NSW.

The incidents have alarmed Communications Minister Anika Wells. She has requested talks with representatives from Optus’s parent company Singtel, who are in Sydney this week.

Singtel’s share price dropped more than 2 per cent on Monday.

Opposition Leader Sussan Ley criticised Prime Minister Anthony Albanese for being in Britain while the “Optus crisis is getting worse here in Australia”.

“The entire triple-zero ecosystem needs an urgent inquiry, not just by the regulator, not just this tip-toeing around by the government,” she said.

-with AAP