The Health and Disability Commissioner Anthony Hill has found a doctor breached the rules protecting a patient's rights by prescribing the person five times the normal amount of methadone.
The names of the doctor and the patient have been withheld.
The incident in May 2011 involved the Southern District Health Board.
The DHB was also found to have failed the patient, who was addicted to opiates and on the methadone programme at the time.
The incorrect dosage arose when the patient confused milligrams with millilitres when giving the doctor details of the dosage
An attempt on the day to verify the correct dosage was unsuccessful but instead of informing the next shift of this fact, the doctor left the patient's chart with the incorrect dose on it.
The patient was treated for an overdose in the hospital's high dependency unit, made a full recovery and was discharged two days later.
The commissioner recommends the doctor and board apologise to the patient and introduce better systems to help with information sharing.
Waitemata District Health Board director of clinical training Pat Alley provided expert advice to he inquiry and describes the mistake as a close call.
Professor Alley says the doctor should have obtained independent verification of the dosage before including it on the patient's chart.