One of the major questions would be how the abuse was able to continue for such a long time.
Care would be taken not to conflict with the police investigation, Mr Forbes said.
Staff and a range of other people with connections to the school were likely to be interviewed, but no children. Other schools were Parker taught - understood to be Oturu and Awanui - and community groups are likely to be included.
The investigation was due to be completed by the end of term, in three weeks' time, and the report two weeks later when the new term started. That could be extended, depending on what emerged from the interviews.
Mr Forbes believed the report's findings would be shared publicly. "What can be shared needs to be shared so there is a sense of learning that comes out of this. That [Parker's offending] was the major crime - it would be another crime not to learn from what went on.''
Support would continue to be available at the school for those who needed it. The revelations had created huge shock and surprise at the time but staff and children were getting on.
"I've been amazed at the resilience of the teachers, in their ability to focus on what's best for the students,'' Mr Forbes said.
Parker is due to be sentenced on November 15.