Up to 55 patients could be affected by a batch of equipment that was not properly sterilised in Hawke's Bay, this month.
Hawke's Bay District Health Board chief executive Kevin Snee told media today the equipment was used in operations between February 2 and 11.
While the batch of equipment was cleaned under hot water and dried, it did not pass through a final sterilisation process before being distributed for use.
It was a diverse tray of equipment, used in both theatre and the hospital's community block. A wide range of procedures would have been carried out, including dental.
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Patients may have been exposed to infection, although advise given suggests it is remote, Mr Snee said.
"My priority as a doctor is patient care ... I wasn't happy when I found out, how could I be when so many patients are affected, but what we have to do is keep a clear head and deal with the issues as we find them and make sure we protect our patients.
"The risk is very remote, but even very remote risk is one that's unacceptable."
He apologised for the stress the situation may have caused.
All 18 theatre patients which may have been affected have been contacted, and the rest are expected to be contacted today. Patients will be invited to the clinic to explain what happened to them, as well as discussing a care plan.
The DHB's Dr Colin Hutchison said patients have been "very understanding" so far.
"We know the names of everybody that's potentially at risk and we are contacting everybody. There is no need for the general population who may have had a procedure in Hawke's Bay Hospital to be concerned."
An event review into what happened will take place, and staff had been briefed to make sure it would not happen again.
When asked by media if the issue was caused by human error, Mr Snee said it was likely several things played into the mistake.
The problem was discovered on Monday afternoon by a theatre nurse who noticed an unsterilised pack of equipment and alerted her managers. Affected equipment was recalled before more than half was used.
Patients who may have been treated with the equipment will be treated based on their individual needs, but would include taking blood tests this week, in six weeks time and then again at 24 weeks. Any infection will not become clear until the final test.
The main diseases that could have been contracted are hepatitis C, hepatitis B and HIV.
Patients affected would be advised to get a vaccination for hepatitis B. Their family will not be at risk of contracting any disease.
The DHB had looked back through a log and found this to be the only case where the sterlisation had not been completed.