The woman, who said her shocking memory might have been due to a stroke in 1995, had had numerous hospital admissions.
A CT scan report clearly indicated her gall-bladder had been removed, stating that the clips from the operation were clearly seen.
The surgeon viewed the report on his computer and tried to print it out, but it was early evening and the printer was in a locked office.
Afterwards, he could no longer find the report on his computer and he did not seek IT help. He expected to receive a hard copy, but said it never arrived.
When the surgeon next saw the patient, he made no mental connection between her and the CT scan report.
The woman's hospital files referred to the 1996 operation. However, they were in two volumes, of which the first provided the bulk of information about the operation, and that volume was not supplied to the surgeon. Nor did he request it.
He had just the second volume, which he later noted contained only a brief reference to the 1996 surgery.
Mr Hill said the DHB should have had a system to alert doctors to the existence of relevant information.
He said an earlier ultrasound scan - which had found the gall-bladder was "not seen and may be contracted" - and blood-test results should have prompted the surgeon to review his diagnosis.
The surgeon had "recognised his error" and had made a written apology to his patient.
After the unnecessary operation, in 2009, the woman was sent for corrective surgery by a Waikato DHB surgeon in Hamilton, where it was found the extra-hepatic bile duct had been removed. The woman, who had a long history of abdominal problems, had a "difficult and protracted recovery".
Mr Hill found the original surgeon had breached the Code of Patients' Rights and referred the case to the Director of Proceedings to decide whether to take him to the Health Practitioners Disciplinary Tribunal.
Waikato DHB said it had put an action plan together in the days after the operation to ensure the mistake was never repeated.
Chief medical adviser Dr Tom Watson extended his apologies to the woman and her family, saying that he and the surgeon had met them regularly since the operation.
"The family have supported our actions because they, like us, want to ensure nothing like this happens again."
Dr Watson said "the process from there was one of learning and not blame".
He said Waikato DHB sends out more than 28,000 records a month for outpatient clinics, emergency departments, wards, queries, research and audits.
"We are embarking on a project to start scanning more information from clinical records into the electronic records but it will take many years," said Dr Watson.
The DHB had designed bright stickers to go on patients' clinical records if incomplete.
These say that diagnostic results and some documents may exist only in the electronic clinical record, and refer clinicians to this.