A major outage at Australia’s largest telecommunications company has led to the cancellation of train services, left thousands of customers without mobile coverage and sparked an investigation into unconnected emergency calls.

Telstra chief financial officer Michael Ackland apologized for the issue which began at 4.30am local time on Wednesday and affected “some mobile calls and data services”.

Services were fully restored about 12 hours later, he said. A software defect related to timing servers in the Sydney and Melbourne data centers was to blame, not a cyberattack, Ackland added.

Australian Prime Minister Anthony Albanese said the outage was “deeply worrying”.

Telstra described the outage as “intermittent” but acknowledged the impact had been “national”.

Ackland said the telecommunications company had carried out welfare checks on customers who had called emergency services during the outage, with six of them requiring immediate help.

Backup systems, which route emergency calls through other mobile operators, largely worked as they should, he added.

Asked if the country could still rely on its largest mobile network, Ackland said: “Australia can absolutely have faith in its largest telco… we take these outages very, very seriously.”

“Our investment in resiliency, cybersecurity and redundancy in our network is significant, but it is a large and complex network and, from time to time, problems occur.”

confirmed that social checks were being carried out for around three dozen calls to emergency services that were not made, but that “the central triple zero system remains operational”.

Communications Minister Anika Wells said the country’s telecommunications regulator, the Australian Communications and Media Authority, will investigate the outage.

In Victoria, all regional train services were canceled due to the disruption, while some regional services in New South Wales were also disrupted. National cargo services were also affected.

Payment systems also went down and around 80,000 companies using the Tyro app were affected.

Last September, a system outage at Optus, Australia’s second-largest telecommunications company, led to three deaths after hundreds of people in more than half the country were unable to call emergency services for 13 hours.

Optus was also fined after an outage in 2023 left thousands of people unable to call emergency services.