An overworked doctor gave a patient a drug to which he was allergic without asking about allergies or following clinic policies.
Health and Disability Commissioner Anthony Hill said on Monday that the unnamed doctor breached patient rights.
A 60-year-old man went to an unidentified accident and medical clinic in 2012 with problems related to a bacterial skin infection.
He was allergic to penicillin but was given a penicillin-based antibiotic by a busy doctor at the end of a night shift.
The patient had a reaction the next day and, though unharmed, needed observation in hospital.
A medical expert said it could have been life-threatening if he'd had an anaphylactic response, which causes the airways to tighten.
Mr Hill says the doctor failed to ask the patient about allergies or to follow clinic processes that would have alerted her to them.