Glenn Wright, then 20, was attending a training course at Optus premises in Gordon in March 2001 when a co-worker he had only just met attempted to murder him, later saying that he had a desire to kill someone and had settled on his victim the previous night.

Mr Wright managed to escape but suffered a blow to the head and later developed chronic severe post-traumatic stress order as well as anxiety, depression and social phobia.

Although he was employed by a labour hire company at time, Mr Wright successfully sued Optus Administration Pty Limited, a subsidiary of Optus, for $3.9 million in damages in 2015.

The primary judge ruled that Optus owed Mr Wright a duty of care analogous to that owed by an employer to an employee and that duty extended to protecting him from the criminal acts of others in the workplace.

But in the New South Wales Court of Appeal, Optus contended that they did not have any duty of care not to cause Mr Wright mental harm and it had not been reasonably forseeable that his life would be placed in peril.

The primary trial heard that Mr Wright's attacker Nathaniel George had left the training room without permission and shortly afterwards sent two messages via other workers to him inviting him to join him upstairs.

The course leader went to look for Mr George and found him wandering in a trance-like state on the roof top balcony, uncommunicative except for asking "Where's Glenn?"

The leader formed the view that Mr George was on drugs, and went to her colleagues for help.

One of them assumed that Mr George and Mr Wright might be friends and so asked for Mr Wright to be brought to the rooftop hoping he might be able to help.

Mr Wright alleges that in the lift to the rooftop he was told: "Nathaniel is up on the fourth floor and appears to be psychotic or on drugs. He is asking for you, do you know what this is about?"

Mr George lured Mr Wright to the balcony edge and invited him to look over the railing at the car before he grabbed him and tried to lift him upwards before other workers intervened.

The trial heard that Mr George later told a worker he had only had three hours of sleep and had thought about the murder all night, planning to ask Mr Wright if he liked the view to get him close to the balcony rail.

"I'm prepared to deal with the consequences. They might give me 10 years. I just want to know what it's like to kill someone. My heart has been going. It's a rush. I wanted to see him lying on the ground dead," Mr George said.

On appeal, Optus argued that Mr George's behaviour suggested only that he was a risk to himself not others and challenged the primary judge's finding that the reasonable response would have been to remove Mr George from the premises and not allow other workers to approach him.

Justices John Basten and Clifton Hoeben allowed the Telco's appeal, ruling that it was not probable that any of the Optus staff knew or should have known that Mr George might attempt to kill Mr Wright and none of them owned Mr Wright a duty of care with respect to the mental harm.

"To conclude that no person should have been permitted to approach Mr George ... or, finally, that the one person he had asked to speak to should not have been allowed to speak to him, involved a number of unwarranted steps and bears the hallmarks of benefiting from hindsight," Justice Bastenwrote in his judgment.

In a dissenting judgment, Justice Fabian Gleeson rejected Optus' argument that there was nothing that suggested Mr George would be a danger to others.

"It was reasonably foreseeable that a person found on a roof balcony, unauthorised, behaving as though severely affected by drugs, and disobeying instructions and directions from his workplace supervisors, might harm himself or others who came in proximity to him at or near the balcony edge," he wrote.