A major outage at Australia's largest telecommunications company has led to cancelled train services, left thousands of customers without mobile coverage, and sparked an investigation into emergency calls that were not connected.

Telstra's chief financial officer Michael Ackland apologised for the issue, which began at 04:30 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 time-keeping servers at data centres in Sydney and Melbourne was to blame - not a cyber attack, Ackland added.

Australia's Prime Minister Anthony Albanese said the outage was "deeply concerning".

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

Ackland said the telecoms company had conducted welfare checks on customers who had called emergency services during the outage, with six requiring immediate help.

Back-up systems, which divert emergency calls to other mobile carriers, largely worked as intended, he added.

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

"Our investment in resilience and cyber security and redundancy in our network is significant, but it is a big and complex network, and from time to time, issues do occur."

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

In Victoria, all regional train services were cancelled due to the outage while some regional services in New South Wales were also disrupted. National freight services were also affected.

Payment systems were also down, affecting about 80,000 businesses using the Tyro app.

Last September, a systems outage at Optus - the second largest telecoms company in Australia - led to three deaths after hundreds of people across 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 unable to call emergency services.