Australians across the nation were unable to make calls to triple-0 for more than two hours early on Friday morning after a cable was severed in rural New South Wales.

Telstra has announced the emergency service is “returning to normal”, but outrage has surged among health work advocates who stress lives may have been lost.

“There will be no way of telling how many people may have been affected by this, but I can’t believe there is no built-in safeguard,” Health Services Union secretary Gerard Hayes told The New Daily.

Mr Hayes said those caring for patients in need of immediate care would have also been thrust into a chaotic situation.

“Stress levels are already high; then you ring on a number you imagine will be your lifeline and suddenly that number is dead. People around an individual would go into panic mode,” he said.

Intermittent disruptions affected emergency calls in New South Wales, Victoria, Queensland, South Australia, Western Australia and Tasmania.

The cable, which is one of three major fibre links across the nation, was restored just before midday on Friday.

Telstra said the damage to the cable running between Orange and Bowral was caused by an “unknown party” and believes it was “consistent with [a] lightning strike”.

A Bureau of Meteorology spokesperson told The New Daily in a statement the agency’s lightning detection systems observed a band of lightning strikes sweeping through inland NSW overnight.

“But they were predominantly confined to areas west and south of Orange,” the spokesperson said.

Telstra chief executive Andy Penn said he was “deeply” sorry for the disruption at a press conference on Friday.

“We had a particular set of circumstances here which has affected some services and for that I deeply apologise and I know critically in relation to triple-0, that’s a critically important service,” Mr Penn said.

But the apology has failed to temper outrage, with social media users calling the service failure “unacceptable”.

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NSW Police Assistant Deputy Commissioner Mark Walton told the ABC emergency services realised around 2am that calls were not getting through to triple-0.

Mr Walton said back-up systems kicked in from 4am, but couldn’t say how many calls had been missed in NSW due to the outage.

“We really don’t have any numbers on that at the moment,” he told the ABC. “We have asked that question of all our agencies. That’s something we’ll assess.”

People facing emergencies were told to call the Police Assistance Line – 133 444 – if they couldn’t access the triple-0 service.