A Royal Commission into Defence and Veteran Suicide was told Private Shaun Jenkins' family had noticed significant changes to his "normal jovial, cheeky" personality when he joined them for Christmas in 2015 after returning from his seven-month deployment.
"He was very withdrawn ... he would isolate himself for fairly significant periods of time in his bedroom," Shaun's father, Peter Jenkins, told the commission on Thursday.
"He was very quiet, very different to what he had been prior to deploying."
A month later, on January 31, 2016, Shaun took his own life.
Struggling to speak at times, Mr Jenkins recalled his final conversations with his son when he told him he was getting less than four hours sleep a night.
Mr Jenkins also noticed his son was highly agitated "which was very rare for Shaun".
He said his son had been tormented after witnessing the death of a 16-year-old girl in Afghanistan. He was also "very upset" over a military vehicle - Bushmaster - accident which left him unconscious and a number of others injured.
"He would lie awake in bed thinking of what he had seen and what had happened in front of him ... in Afghanistan," Mr Jenkins said.
"He was in a constant stage of suffering mental anguish over his deployment."
Mr Jenkins said it reached the point where suicide felt like the only option "to stop the pain".
"They actually believe that their loved ones and family members will be better off as a result of that," he said.
Mr Jenkins, a retired Queensland police officer, said trying to find out what had happened to Shaun, including seeking medical records from the defence force, had been a "horrendous experience".
He said Shaun's mother, Sue, contacted the Department of Veterans' Affairs to see her son's medical records, but the request was refused.
"The first thing they said to her was that she was not entitled to any money or compensation because she wasn't his partner or child," Mr Jenkins said.
"We didn't want money ... as parents all we wanted was answers as to why we had lost a son. We just wanted answers. We wanted to find out what had happened to our son."
Mr Jenkins told the commission the DVA urgently need to train its staff to be more "empathetic" and stop behaving "like it was nothing more than a large insurance company that was looking at protecting its assets rather than providing any assistance or support".
He said the ADF had also refused to return their son's mobile phone and failed to alert them a coroner's report was being prepared on the circumstances surrounding Shaun's death.
It wasn't until the family received the report in May 2018 they discovered the army had been aware their son had been suffering from suicidal ideation and had been Googling ways to kill himself. But he was not considered at risk.
Mr Jenkins said urgent changes were needed in the defence culture so people like his son felt more confident to say "I've got a problem".
Better education of leadership about the mental health red flags to watch out for, and post-deployment support for all veterans was also critical, he added.
"Simply asking the question if 'you're OK' is never going to be sufficient for people that have been in those types of situations," he said.
The commission's public hearing continues.
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