More than 11,000 Sydney dental patients may have been exposed to HIV and hepatitis because of unhygienic practices at four clinics across the city.

New South Wales (NSW) health authorities said on Thursday the disease risk was caused by poor cleaning and sterilization techniques at the four dental practices.

The issues at the four practices go back a number of years, NSW Health said in a statement.

The department is recommending people who had invasive procedures at the Gentle Dentist in Campsie and Sussex St. or the Surry Hills and Bondi Junction surgeries of Dr. Robert Starkenburg be tested for blood-borne diseases.

Gentle Dentist operator Dr. Samson Chan, four of his staff and Dr. Starkenburg have all been suspended following investigations prompted by customer complaints.

"There is an array of problems, including the cleaning of instruments, functioning of sterilizers, the knowledge of the practitioners and recording of the results of the sterilizing equipment," Dr. Jeremy McAnulty, director, health protection at NSW Health told reporters on Thursday.

Another six dentists from the Gentle Dentist clinics have had conditions placed on their licenses.

NSW Health says there is no evidence of transmission and chances of infection are low.