A former police officer was subjected to extensive physical and video surveillance by an insurance company, even after its psychiatrist diagnosed him as "badly depressed, if not at risk of suicide" and struggling with severe post-traumatic stress disorder (PTSD).

Key points: Former police officers spied on by private eyes working for insurance firms

Former police officers spied on by private eyes working for insurance firms Psychiatrist warns surveillance can make PTSD worse

Psychiatrist warns surveillance can make PTSD worse Lawyer says such actions could in fact be illegal

The distressing encounters which Brendon Bullock and other police officers suffering from PTSD have had with insurers and private investigators are detailed in tonight's Four Corners program, Insult to Injury.

The surveillance includes intrusive monitoring of social media accounts and other online activity. In the case of one former officer with PTSD, investigators downloaded 13,427 items.

A former general duties officer, terrorism investigator and undercover operative for the Australian Crime Commission, Mr Bullock witnessed many traumatic events in the course of his duties — car accidents, suicides, homicides and other extreme acts of violence.

The final trigger for his mental illness was attending the scene of a murder where he found the killer and his badly mutilated victim.

Mr Bullock left the police force in 2011 and submitted a compensation claim for his illness under a scheme offered by New South Wales Police Force's superannuation fund and managed by insurer MetLife.

In December 2012, a psychiatrist selected by the insurer assessed him as having a "total and permanent" disability because of his acute PTSD. The report was given to MetLife, although not to Mr Bullock when he asked for it.

Despite the report, and entreaties from his lawyers for the surveillance to stop, MetLife employed investigators to watch him on another 10 occasions over the next seven months. On four of those occasions they videoed him as well.

"He spiralled downwards after [the surveillance] started to happen over that period, unbelievably rapidly," said Mr Bullock's father Bruce. "He was in a really, really, bad way."

"At that particular time I was in a state of self-destruction," Brendon recalls.

"I was drinking heavily, I was heavily medicated. I was hyper-vigilant, very aware that I was being watched, yet couldn't understand why this surveillance was continuing."

In November 2013, Mr Bullock attempted suicide, his depression exacerbated by the surveillance and news that MetLife would no longer handle his claim and another insurer, TAL, would take over. He was found comatose by his wife and daughters.

One in five officers at 'significant risk' of mental illness

Brendon Bullock tried to take his own life after being placed under surveillance. ( ABC News )

Psychiatrist Sandy McFarlane, from the University of Adelaide's Centre for Traumatic Stress Studies, said surveillance worsens an officer's PTSD.

"If a person is being followed it very clearly can make them significantly worse because they perceive this threat in their environment that brings back very real things that have happened to them in their career," he said.

The surveillance especially unnerved Mr Bullock because he twice received death threats when he was a policeman.

Professor McFarlane estimates one in five police officers are at "significant risk" of mental illness due to the stress of their work.

He said insurers should not view PTSD as "simply a problem of compensation".

In response to queries from Four Corners, MetLife announced it had changed its procedures and "now no longer use surveillance in the assessment of claims related to mental illness".

Email, social media use reveals 'hobbies': investigators

Private investigators scoured Greg's social media accounts. ( ABC News )

Another former police officer diagnosed with PTSD, who asked to be identified only as Greg, was placed under lengthy surveillance by private investigators hired by another insurer, Employers Mutual.

Their work included what is known in the industry as a "desktop" investigation — trawling through the internet activity of claimants.

Investigators scoured Greg's social media accounts and downloaded 13,427 items in less than a month.

In a report compiled by Lee Kelly Commercial Investigations on Greg's internet activity, which was obtained by Four Corners, the investigator says they were able to mask their visits to social media sites.

"Our collections of monitored accounts are passive and do not announce 'follows' to target accounts as is the case when regular approaches are employed," it said.

Greg's online conversations with his lawyer and frequent posting on sites related to police officers' mental health were viewed suspiciously by the investigator who considered them "inconsistent with assertions made by the claimant". The investigator said the activity showed he had "concentration and persistence", as well as "interests and hobbies".

When Greg changed his Facebook picture to a monkey smoking a cigarette, the investigators reported that it appeared to be "an attempt at humour and to thwart surveillance".

Investigators acting illegally, lawyer says

Lawyer John Cox said the online investigation technique had become popular in the past 18 months.

"[It is] not only intrusive but, in my opinion, the actions of some of these investigators are illegal," Mr Cox said.

"Social media is important for many ex-police. It is the only way that they socialise," he added.

"The law says [in] Section 308H [of the NSW Crimes Act] that anyone who breaches a security setting on a computer system is committing an offence."

Greg said his Facebook account had the highest privacy setting when the investigators began their probe in April 2015. "It could only be viewed by friends," he said.

Lee Kelly Commercial Investigations said in its report it did not breach privacy legislation when collecting data, but declined to comment on any breach of Section 308H when contacted by the ABC.

Employers Mutual also declined to comment, referring inquiries to the NSW Workcover authority, now known as Insurance & Care NSW, or icare.

In a statement, it said insurance fraud was a "serious issue" and it had been "advised that all information in relation to [Greg's] claim was obtained from publicly available information".

Surveillance logs obtained by Greg show he was followed for 80 hours by private detectives, who took video and photos of him with his family at the shops and walking along a beach.

It was, says Greg, "absolutely humiliating".

Watch Four Corners: Insult to Injury at 8:30pm on ABC TV