Twenty-three survivor witnesses, their whānau, or staff at institutions will give evidence about their time in care or working at care facilities throughout the country between 1950 and 1999.
They have described the hell they endured from disabled girls being raped while boys were forced to watch, to brutal assaults, over-medication and a level of dehumanisation akin to concentration camps.
Alison Pascoe, now 80, has never had a mental illness and said she should never have been placed into care.
Growing up in a violent household, a severe bout of chickenpox saw her shipped to a health farm with 300 children.
Pascoe was then committed to a psychiatric institution at the age of 8 to 48 years (1955-1990).
The abuse Pascoe suffered impacts her to this day with one nurse particularly sadistic in her treatment.
"She tried to make me eat my own faeces, drink my own urine, that's not normal in a job like that is it?"
She said the neglect and abuse could be fatal and said she often bore witness to murder by medical overdose.
As with many other survivors, Pascoe spoke about the need for accountability and change.
"I could never forgive them and never forget what happened."
Dr Olive Webb worked at Sunnyside Hospital for 28 years.
"None of the staff were adequately trained. They did not really know how to care for people, they only knew how to control them."
One of the wards she was responsible for housed 70 men.
She said every day the men were hosed down by staff before being marched back through the villa.
"Concentration camps come to mind."
Webb said the men were then herded into a room together where days were timeless.
"And there they sat, and did nothing. It was a complete removal of thinking, creativity, dignity and independence."
Allison Campbell started working for IHC as a social worker in 1980 and continued to work for IHC in various roles until she retired in 2002.
In her statement on Monday, she talked about the abuse and neglect she witnessed in both psychopaedic hospitals and in IHC care.
"I think New Zealanders should be ashamed. We should be ashamed as a nation. For instance, they look down on people, they felt that they were subnormal and they were certainly treated like animals. And if animals had been treated that way, they would have been charged."
Sidney Neilson was at Porirua Hospital between the ages of 19 and 42 and given shock treatment every day without consent from neither him nor his whānau.
"It was hell, it was worse than prison."
He was exposed to terrifying levels of violence between patients and even police could not or chose not to intervene.
During one fight in the kitchen, he said one man cut another man's eye out with a knife.
With almost all patients Māori or Pasifika and all staff Pākehā, racism and cultural oppression was also systemic.
Catherine Hickey spoke to the commission representing her brother, Paul.
Paul was hit by a truck when he was 6 and as a result had a serious head injury and was paralysed on one side of his body.
After an assessment, Paul was made a ward of the state and committed to hospital when he was 15.
Hickey read a letter to the commission that her brother had sent her in 1976.
It read: "I don't want to upset you, but the staff do terrible things to me. I don't want to go for a shower anymore because I can't protect myself and can't tell you what they do."
Another letter submitted as evidence was from a medical professional who stated that Paul would be better off dead.
Just one year later, Paul took his own life.
"His abusers were protected by the state, despite our complaints, his torture continued."
