Although no official records were kept it was thought the number was around 200 young people, but now the number is believed closer to 300.
Lead investigation counsel Andrew Molloy said the unit had achieved a degree of notoriety over the last 40-50 years without ever being the subject of a rigourous overall examination.
"There was some litigation back in the early part of this century which saw the Crown compensating some of the children who had been in the unit, having accepted that the kind of treatment they were subjected to at that time was inappropriate, even by the standards of the day.''
For many of the survivors there has been an ongoing concern that there had not been appropriate accounting for what happened to them, he said.
''It's not just a fiscal thing, it is a question of accountability of why what happened happened and why hasn't anyone been held responsible for what happened.
''The purpose of our inquiry is to look at it in a very broad sense, some of the previous investigations have been a little siloed. We have the opportunity to look at it across a broad range of fields.''
The commission had a lot of documents it had assessed and had spoken to a lot of people, Molloy said.
Forty witnesses, of which about 20 are survivors of the unit, are expected to give evidence at the public hearing, which will take 12 days.
''I think we will hear from people who will talk about what has happened to them in the past and what has continued to happen to them since, in a way that will be fairly confronting for most of us I think.''
Other witnesses will include former staff members, some medical experts, such as psychiatrists who were not working at the unit at the time but were contemporaries.
There will also be police witnesses and Solicitor General Una Jagose QC.
Molloy said there were a number of factors that made what happened in the unit at Lake Alice different from other similar institutions.
''It's not simply that it was a different time. It was remote from other people and units, it was out in the countryside. The other child and adolescent units at the time were adjacent to metropolitan hospitals, that was one of the factors. It seems to have fallen through the cracks in terms of checks and balances.
''The exact reasons will coalesce as we pull the strands together, but I don't want to preempt any actual findings about that, that is really part of what we are doing is to try and understand that and certainly the public hearing will be part of that process.''
RNZ
