A newly employed nurse mistakenly used the same needle on 55 people at a New South Wales clinic.

More than 50 cancer patients and two staff members, who were tested at the Gosford clinic between November 28 last year and January 28, are now being tested for HIV and hepatitis B and C.

The nurse made the blunder while using the Accu-Chek Multiclix, a device used to check blood sugar levels.

PRP Diagnostic Imaging chairman Dr Michael Jones told media the nurse mistakenly believed the device automatically changed needles when instead the needle should have been changed manually for each patient.

The error was uncovered when a staff member with diabetes asked the nurse to perform the test on her.

"The moment we found out, we withdrew it (the device) from service," Dr Jones said.

The clinic, which is part of the largest private radiology company in Australia, has now changed to single-use devices to prevent the mistake from reoccurring.

Patients have been sent letters of apology, urging them to undergo blood tests for HIV and hepatitis B and C.

Dr Jones said the chance of infection remained "low or very low".

"It was very regrettable and we are extremely distressed it happened," he said.

The patients visited the clinic for a positron emission tomography (PET) scan used to determine the severity of cancers, neurological conditions and cardiovascular disease.