Leading Aircraftman Gary Dale Sargeant, 36, was a "valued and loyal" defence force member who had twice toured the Middle East.
In findings delivered on Thursday, South Australian coroner Naomi Kereru said there were a number of occasions when action could have been taken to prevent his death.
Mr Sargeant was posted to the cyber vulnerability investigation team at RAAF Edinburgh in 2020, but experienced social isolation while working from home during the COVID-19 pandemic.
The "introverted and quiet" man had a history of severe depression, anxiety, suicidal ideation and alcohol issues, with "a background of childhood trauma which he struggled to overcome", Ms Kereru said.
In early September 2020, Mr Sargeant visited the RAAF health centre where he told a nurse that when he was younger he would fantasise about suicidal acts.
"He stated he never acted on these thoughts but would cry himself to sleep nightly and promise himself things would get better," the nurse's referral stated.
The urgent referral was sent to independent psychologist Kathleen Mansfield, who saw him a few days later, but she was unable to open his password-protected file.
"The failure to read this information highlights a lost opportunity to provide a perfectly timed intervention," Ms Kereru found.
Ms Mansfield's failure to read the referral prior to seeing Mr Sargeant was "inappropriate" and her approach to the case was "lacking", she said.
On September 9, 2020, Mr Sargeant suffered an acute mental health episode and went to Edinburgh's military police station, where officers formed the view that he was paranoid, unwell and distressed.
He was subsequently assessed at the base's health centre and detained under the Mental Health Act until an ambulance could take him to hospital.
However, there was no secure area at the centre, and after Mr Sargeant asked a nurse to get him a phone charger, he exited the building and left the base in his car.
That evening, South Australia police found his car at the North Haven Surf Live Saving Club, and the following day, his body was found on breakwater rocks at Outer Harbor.
A post-mortem confirmed his cause of death as drowning.
The death could have been prevented if he had been searched and had his car keys removed at the health centre, or if a guard had remained with him, Ms Kereru found.
He had "longed to reconnect with his distant family", she noted.
"It is especially unfortunate that he never had the opportunity to realise that deep desire," Ms Kereru said.
She recommended military and federal police be given the power to detain and search people with a suspected mental illness, and military health facilities be fitted with a secure treatment space.
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