The Nelson Marlborough District Health Board has been found to have breached a patient's rights by giving him more than the recommended quantity of medication and failing to record the doses.
The Health and Disability Commissioner was deciding on the case of a man who was admitted to the DHB's acute mental health care unit in 2008.
After being admitted, the 64-year-old man was injected with amounts of fluphenazine above the manufacturer's recommended dosage.
The man's physical condition declined and he died the next year.
Commissioner Anthony Hill says while he did not assess the cause of the man's death, he found the DHB breached the patient's right to service of an appropriate standard.
Mr Hill has recommended the DHB apologise to the man's family.