A man needed seven operations, a large amount of his scalp removed and skin grafts after a mistake by a family doctor.
Health and Disability Commissioner Anthony Hill said the unnamed GP failed to give important details to his patient.
The GP removed a scalp lesion and told the 46-year-old if he heard nothing back, all was well.
A low-grade cancer was diagnosed, but the GP failed to read the histology report fully, including the comment that a follow-up was advisable.
The need for a follow-up was not raised with the patient, who did not know he had aggressive skin cancer, until well over a year later, when the lesion reappeared and he consulted a specialist.
The man survived, but had to have a large amount of scalp removed, down to his skull.
Mr Hill said, had the man known about the laboratory findings, he could have sought treatment earlier.
The GP, who has retired, was told to apologise.