Gina Tatto said the 2018 tragedy "would not have happened" if her brother Joshua Barker received the treatment and medical assessment he so desperately needed.

Mr Barker was taken to the Launceston General Hospital by police, after his mental health began to rapidly decline and his sister called triple-0.

He was discharged within half an hour, with no treatment or followup, according to his sister.

The family continued to try and seek help from mental health services to no avail.

Four weeks later, the 30-year-old ran down Prospect Vale man Dale Watson with his car, killing him.

But he has little memory of that day, or the week leading up to it.

During his trial, it was revealed Mr Barker had been savagely attacked by a group of men when he was 22, causing both physical and long-term mental damage.

He was later diagnosed with post traumatic stress disorder, suffering flashbacks, anxiety attacks, and intrusive thoughts.

He had no criminal history prior to the tragedy, and was ultimately found not guilty by reason of insanity.

But the verdict was not until he had already spent two years behind bars, instead of at a mental health facility.

"The system has failed Josh, over and over, and we have lost all faith in the process," Mrs Tatto said.

"Josh has lost two years of his life, when he could have been getting treatment. He should have been placed in the Wilfred Lopes Centre at the very beginning, he should never have been dragged through the justice system and court.

"We were dragged through it too, and so was the Watson family.

"He still has no memory of it all, and is in disbelief. Imagine waking up in a prison cell and having no clue how you got in there, he was petrified."

The Barker family is now calling for an overhaul of the state's mental health system.

"The system is so broken, and we are going to keep seeing this happening until there is change," she said.

"There needs to be a review of how people access mental health services in Tasmania. How many more lives have to be lost before there is change?

"Josh is not just my brother, he is my best friend, I feel like half of my heart has been taken away. I will keep fighting for Josh, until there is change. What Josh and our family have been through, I never want anyone else to go through."

IN OTHER NEWS:

The case was featured on ABC's Four Corners program on Monday night, as part of an investigation into mental health systems across the country.

But Mr Barker was just one of many examples of the system failing in Tasmania, according to his lawyer Evan Hughes.

"It is something that has been a problem in our state for as long as I have been a legal practitioner, and that is over 20 years," Mr Hughes said.

"The difficulty is when the mental health system is not working properly, the criminal justice system is the system that ends up dealing with the aftermath."

With vulnerable clients fronting court almost daily, Mr Hughes said the stigma linking mental health and crime with drug use also needed to end.

"The case of Joshua Barker was not drug related, we are talking about people purely suffering mental health problems," he said.

"I would like to be able to say Josh's case is isolated, but it is not, I have had other cases, some very recently and very similar, which have involved intelligent, articulate men who have become very unwell, very quickly and have caused physical harm to family members.

"The criminal justice system is not a replacement for the mental health system, it is designed to rehabilitate and ultimately to punish, it is not designed to deal with significant and serious mental health cases."

While there were a number of "gaps in the system", Mr Hughes said overall, there needed to be better support in place for not only those suffering these illnesses, but their families and close contacts.

"The biggest takeaway for me from dealing with what happened to Dale Watson, was it was so preventable," he said.

"Josh's family fought very, very hard and consistently to try and get him help, and they will tell you they failed.

"What hope does a homeless person, or a disconnected person, or a young person have, if Josh's very capable family can't get help? It is deeply troubling."

Mental Health and Wellbeing Minister Jeremy Rockliff said the state government had already begun reforming services in the state, after a report from the Mental Health Integration Taskforce was released last year.

"I would like to acknowledge the courage of the families of both Dale Watson and Joshua Barker in contributing to the Four Corners program," he said.

"The recommendations and issues raised by the taskforce are informing a new approach to improve the integration of mental health supports and services.

"The Tasmanian Government is committed to increasing access to mental health services and enabling better recovery outcomes for Tasmanians living with mental health challenges, their families and friends."

Mr Barker was ordered by the court to remain at Wilfred Lopes Centre indefinitely.

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