The boy was due to have a tonsillectomy and adenoidectomy surgery, and his sister, aged 4, was due to have the same procedures immediately afterwards.
The nurse realised that a mistake had been made only after she checked the sister's prescription, which was for 8mg of codeine.
The boy had his stomach washed out and had the surgery as planned.
The commissioner considered the nurse demonstrated "very poor judgement" and the actions of the senior nurse were "concerning".
He found both nurses in breach of the Code of Health and Disability Services Consumer Rights for failing to provide services to the boy with appropriate care and skill.
The report also criticised the legibility and comprehensiveness of the anaesthetist's documentation.
The commissioner recommended the nurses and the hospital provide written apologies to the boy and his family and asked the Nursing Council of NZ to consider reviewing the competence of the nurse.
The hospital had also been asked to undertake staff training on the importance of clear, open and supportive communication with patients and their families.