Deputy Health & Disability Commissioner Tania Thomas says an autistic, intellectually impaired boy was able to escape through an unlocked gate and drown in a pond while he was in the care of an independent charitable trust.
Ms Thomas says Julian Stacey, 11, had the functioning of a one or two-year-old and a history of wandering away from both his home and respite care.
She says his mother had been assured by staff at a facility run by the Spectrum Care Trust Board that her son would be watched closely.
Ms Thomas says that did not happen one weekend during 2010 when he escaped, most likely through an outside gate that staff believed was locked although they had not checked it.
She says Spectrum Care has undergone two audits and made many improvements since.
Julian's mother Tarsha Stacey told Morning Report she believes the trust's staff got off too lightly.
Ms Stacey says staff on-duty when her son disappeared should have lost their jobs or been stood down during the investigation.
She said her family received a formal letter of apology from Spectrum Care only this week, two and a half years after her son died.
Ms Stacey said it would have been easier for her family to move on from Julian's death if the apology had come sooner.