His thinking was always clear. He was composed when deciding how to make sure 1 million people celebrating New Year's Eve in the Sydney CBD would get home safely, even in the face of a growing terrorism threat. He was controlled and level-headed when trying to keep a lid on tensions that threatened to spark a repeat of the violent 2012 Hyde Park riot.

Undoubtedly there was an element of anxiety that came with his job; the safety of thousands of people depended on the calls he made. Yet Clarke, a recently retired assistant commissioner and 40-year veteran of the NSW Police Force, always felt that anxiety subside when an operation ended.

That was until 2015, when he couldn't stop feeling like his chest was being squeezed, his mind would not stop racing and his palms continued to sweat.

"You would feel this anxiety and you would try and pinpoint what it was," the former commander of the major events and incidents group says.

"[You'd think] 'once we get that report done, or once we finish this operation I'll be right'," he says. "But that would happen and there would be no relief."

Soon enough, Clarke fell apart. Shadowed by dark thoughts, one day at work in September 2015, he handed his gun to his staff officer and told him "get it out of here".

The decorated officer, who exercised daily and was at the peak of his physical fitness, ended up in hospital, battling debilitating depression linked in part to traumatic experiences he faced on the job.

His sudden departure and mental illness shocked his colleagues. One detective described him as a "true leader, respected in the highest regard by rank, file and senior police alike".

If it could happen to him it could happen to anyone, another remarked.

Eighteen months on, a strong, healthy and happy Clarke, 57, sits at a leafy, inner-city cafe with a bag of exercise gear at his feet.

Now retired, he still enjoys working out with colleagues at the Sydney Police Centre, where unused police cells have been converted into a gym and boxing ring.

"I am out of the cops but I feel like I am not lost to it," the father of three says.

Today Clarke is speaking out for the first time about the depression that forced him out of the police, in the hope it will encourage officers to identify the warning signs.

"I don't regret this happening, I am not proud of it but I am not ashamed of it," he said.

Clarke started his policing career as an eager 16-year-old fresh out of the Redfern academy. He ascended the ranks, ran local area commands in Lake Macquarie, Ku-ring-gai and Brisbane Water before he was promoted to assistant commissioner in 2009.

He rolled out the introduction of Tasers, oversaw security for some of the G20 events in 2014, was involved in the emergency response to the 2014 Lindt cafe siege and along the way earned bravery citations and the Australian Police Medal.

In late 2015, Clarke was crippled with relentless anxiety - feelings that a week's holiday failed to fix. He had trouble concentrating, reading and struggled to find happiness in things that usually brought him joy. Saturdays spent with his children and grandchildren, a weekly occasion he cherished, drew no emotion.

"I would go 'what's wrong?' I know how happy I should be seeing this dream right in front of me and it's not just right," he recalls.

One day in September 2015, after realising he was "no good to anyone", he took off his uniform for the last time and walked out of the Sydney Police Centre.

"I knew I would never be back," he says.

Clarke was diagnosed with depression, showed signs of post traumatic stress disorder and contemplated suicide. There were times when he thought he wouldn't see the day through.

"No doubt some of that links back to experiences in the job but its also a symptom of the sickness," he says. "The stronger those thoughts the more unwell you are."

In a bid to give his family a reprieve, he admitted himself into hospital. With the help of a psychologist, the unwavering support of his wife Catherine - who accompanied him to every psych appointment - and self-determination, Clarke slowly recovered.

He wrote in a journal every day for six months, exercised, gave up alcohol, surrounded himself with friends and family and put his trust in the doctors tasked with helping him.

Asked whether there were any specific traumatic incidents that contributed to his illness, Clarke says yes, but doesn't want to drag them up again. He cites an expression often used by police when explaining trauma triggers: "My bucket was full."

"In the end I felt like I could hardly cope with life, which is ridiculous because I really didn't have a worry in the world," he says.

The job showed him that "fate dealt a wicked hand".

"The crooks can shoot each other all day long and half of them survive," he says. "And then you just virtually see an innocent person's life snatched away and go 'well, that is fate'. For me as I got older it seemed to affect me more.

"You get a greater depth of understanding of the pain people go through. There are definitely things that I could have dealt with better but I don't know if it would have changed anything."

As it did with Clarke, mental illness can creep up late, affecting distinguished officers at the height of their careers and after decades in the job.

Fairfax Media understands three commanders from different squads at the elite State Crime Command have taken extended leave or retired "medically unfit" in the past six months.

While millions of dollars have recently been invested in mental health prevention for police, some senior officers believe the focus is "downwards", rather than including inspector level and above.

One officer referred to the high turnover of health, wellbeing and welfare staff in the force, an underwhelming focus on senior officers' welfare and a lack of genuineness.

"You are expected to look after your staff and that's right and part of the deal. But the other side of the deal is nobody looks after you," the officer says.

However Clarke, who says he always had good bosses and no one to blame, believes depression was something that just happened to him. Since he officially retired from the force in November last year, and walked out of the police centre through a guard of honour, Clarke has been approached by other officers detailing their own struggles.

Clarke attributed an important part of his recovery to a mentor who lived through a similar experience. Now Clarke hopes to move into a similar role with the NSW Police Legacy Back Up For Life mentoring program.

The initiative selected 15 mentors with "lived experiences" to help former police officers dealing with mental or general health issues, substance abuse or the transition from the force to retirement.

Former police commissioner and NSW Police Legacy director Ken Moroney says there needed to be a mutual focus on these transitional arrangements.

"The police force are doing some really good things for serving officers, Legacy is doing a lot with retired officers," says Moroney, who retired from the police in 2007. "The next important thing will be bridging that gap and helping officers as they transition out of police into retirement."

He referred to an expression used by former police commissioner John Avery: "To understand the issue you need to have touched the flame."

"These mentors are retired police who have lived the experience of retirement and understand the stresses of leaving the job and embarking on a new career and something that might be completely different," Moroney says.

Figures published in 2015 by the National Coronial Information System showed 62 police officers across Australia committed suicide between 2000 and 2012. Twenty-five of those were in NSW, the most in any state.

NSW Police say there has been a strong focus in recent years on identifying the warning signs of mental illness and intervention.

There is resilience training from probationary constables upwards, peer support, as well as psychological checks for officers working in high-risk areas - such as child abuse or homicide - and tracking of deployment to traumatic incidents.

Assistant Commissioner Carlene York says there are a lot less people leaving the police "medically unfit" in recent years and the stigma around post traumatic stress disorder had changed.

"I think it is changing, it is a lot better," she says. "But I still think we have some work to do - it is reflective of the community attitude too."