The woman had symptomatic artery disease and claudication (pain caused by obstruction of the arteries) in her right thigh.
She underwent an operation at an Auckland DHB facility to increase the flow to her legs.
The HDC report said the nurses counted items used during the surgery as required, and no items were unaccounted for.
"The surgeons were notified of, and acknowledged, the correct count," the report said.
However, three weeks after the surgery the woman noticed a lump on the left-hand side of her abdomen, and experienced associated pain and unwellness.
She saw her GP, who documented a soft, tender, fluctuant mass in the upper abdomen, which was not something she would expect in a patient following aorto-iliac bypass surgery.
The GP made an urgent referral to hospital where a CT scan showed a mass in the left abdomen measuring 7.9cm by 8.3cm.
As a result the woman made a complaint to the HDC and an investigation was conducted - the findings have been published today.
Hill said the error to be the responsibility of the DHB and all the staff involved in the surgery.
He was critical of the DHB's count policy and discrepancies in training for different teams at the time of events, and considered that improvement in these areas may help to reduce any unnecessary risk and opportunities for error in future.
"The DHB needed to ensure that its system provided [the woman] with safe care of an appropriate standard," Hill said.
"Somehow, that system failed [the woman], and a swab was left inside her abdomen. As a result of this, the surgery caused unnecessary harm and a protracted recovery process for [the woman]."
In the report, the woman told Hill she remained "dissatisfied that there was a retained swab despite a correct swab count".
She said that she is still traumatised by this surgery, and is still suffering with the outcome.
Hill recommended that the DHB mandate that all surgical staff read the Count Policy and ensure that they keep up to date with any changes.
He also advised that the DHB consider how new medical surgical staff will learn the Count Policy and provide the results of its yearly audit, including details of any changes made as
a result and any specific education provided to staff.
In the report, it said Auckland DHB sincerely apologised to the woman for her surgical experience at the hospital, which resulted in a very painful and protracted journey following what should have been a straightforward surgical procedure.
The DHB advised that its clinical teams strive to provide the best quality of care to all its patients, and the surgical team is deeply saddened about what happened to this woman, and feel very disappointed that its investigation did not provide a clear outcome.
Following review of the patient's incident, it was agreed that yearly directorate-wide audits will be completed in February each year, with specific targeted education to be arranged for any areas where non-compliance has been identified.
