Children who lost a parent to suicide were likely to have feelings of "utter abandonment" and often blamed themselves for being a "bad child", the Royal Commission into Defence and Veteran Suicide has been told.
For adolescent children, the impact was just as devastating, increasing the risk that they would also have suicidal thoughts or begin self harming.
Giving evidence on the third day of public hearings in Canberra, Professor Louise Newman, a specialist in perinatal and infant psychiatry, told the inquiry treating a child or teenager for suicide grief counselling was very specialised and complex.
"They might blame themselves or be more angry and wanting to leave the family and run away, but they are very troubled," Prof Newman said.
"Quite young children, four, five or six will say 'I must have been a bad child and behaved in a negative way for my parent not to want to live and end their lives'. Which is really quite a very difficult thing to recover from and grow up with."
The surviving adult family members were often so distraught the child's distress could be compounded, she said, when the surviving parent refused to talk about what happened or forced their children to keep the suicide a secret.
Making a child carry the burden of a "pathological secret... this very disturbing reality they can't understand ...that's very risky".
Professor Newman said domestic violence was also extremely damaging to children's mental health, and the impact of witnessing a parent being hurt was no less damaging than being physically attacked themselves.
Children growing up in any environment that was "characterised by fear, chaos, sometimes even violence" were always on high alert, she said, "because the world and people around you are unpredictable and dangerous".
Pressed by the commission on whether there was any prospect specialist mental health services could be established for the children of veterans and defence members, Prof Newman said: "I am not at all sure that in the current resource crisis around mental health that would be seen as a priority."
In earlier evidence to the inquiry on Wednesday, the commission heard partners and family members caring for mentally ill veterans tended to be treated as an "annoyance" or "completely ignored" by veteran and defence services.
Professor Sharon Lawn, the former commissioner of the South Australian Mental Health Commission, said families had an intimate and critical support role, yet the injured person tended to be "treated in a silo".
Prof Lawn said the Department of Veteran Affairs services routinely kept family members in the dark, refusing to inform them about everything from the psychiatric medications prescribed or danger signs to watch out for.
"They're (family) invisible to the services," Professor Lawn said. "It's like the head is separated from the body. Who do they think actually helps this person?
"It's not just impacting the veteran, it's actually impacting everyone else at home."
The inquiry was told it could often take up to 30 years before veterans were finally diagnosed with serious mental illnesses, like PTSD, by which time "the damage had been done".
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