During the operation, the surgeon found a large amount of scar tissue, and identified the level of the spine on which to operate using an X-ray image intensifier - a standard procedure.
However a subsequent scan in June found that the surgeon appeared to have operated on level 3/4, not level 4/5.
Meanwhile the patient contacted the surgeon again about nerve pain and faecal incontinence.
In July she went to the hospital's emergency department with back pain.
In August she attended a scheduled post-operative review at the hospital and the surgeon finally saw report on the scan that had been taken in June.
He told the commissioner that he was "shocked", as he had used the image intensifier, and the patient had reported improvement after the operation.
However, he did not contact the radiologist who conducted the scan, and did not tell the patient about it, because he "considered that clinical clarification with the aid of spinal steroid injections would be useful to resolve any uncertainty".
The woman sought a second opinion from another orthopedic surgeon through her general practitioner, and the commissioner said: "Subsequent review of the [scan], including by Ms A's subsequent orthopaedic surgeon, who made contact with Dr C about the issue, resulted in a conclusion that Dr C had operated on the wrong level."
The commissioner found that the surgeon "took appropriate clinical measures prior to surgery to identify the appropriate spinal level on which to operate".
"However, it was clear from the relevant MRI scan that decompression of the L4/5 pathological level had not been performed," he said.
"In the circumstances, including Ms A's ongoing symptoms, Dr C is criticised for not seeking further advice from colleagues and/or the radiologist about the interpretation of the scan at that stage.
"Accordingly, Dr C did not provide services to Ms A with reasonable care and skill, and breached Right 4 (1) of the Code of Health and Disability Services Consumers' Rights (the Code).
"Dr C failed to advise Ms A that the MRI report indicated that it was possible that he had operated on the wrong level of her spine, and that he intended to use the steroid injections to seek clarification in this regard.
"This was information that a reasonable consumer in Ms A's circumstances would need to receive to make an informed choice or give informed consent to the proposed further treatment. Accordingly, Dr C breached Right 6 (2) of the Code."
The commissioner recommended that the surgeon "consult with orthopaedic peers and consider adding to his clinical regimen not only screening with image intensification prior to incision, but also screening again once down to the lamina - and report back to HDC, within three months of the date of this report".
He also recommended that the surgeon "undertake a review of his process for providing consumers with information during the surgical consent process and postoperatively", and apologise to the patient within three months.
