A New South Wales (NSW) Health spokesman said at least 11,000 patients of two dental operators were at "low risk" because of poor cleaning and sterilisation practices.
Patients who had invasive procedures at four practices in the Sydney region could be at risk.
Authorities said they are now contacting all of those patients.
HIV and Hepatitis are blood-borne infectious diseases.
The clinics were operated by Gentle Dental clinics at Campsie in Sydney's south-west and  in the central business district, and by Dr Robert Starkenburg at Surry Hills and Bondi Junction.
Local media reports said the 75-year-old Dr Starkenburg, who was stood down by the Australian Dental Council on 2 December 2014, admitted his hygiene practices may have been "lax at times".
Dr Samson Chan from the Gentle Dentist centres had his registration suspended in March this year, along with four other dentists who worked at the practice, according to an Australian Dental Council spokesman.
Conditions, such as ongoing inspections, were imposed on six more practitioners working at the Gentle Dentist centres.
A NSW health spokesman said anyone who had invasive procedures at any of the clinics should consult their doctor about tests for blood-borne illnesses.
"This is really for those people who had an invasive procedure, so not just a regular check up, but invasive instruments into the gum and more complex dentistry," he said.
"The issue is if a patient with a blood-borne virus attended one of these practices and the instruments used were not sterilised properly," he said.
But he said there was a low risk of infection in these circumstances.
Thousands of Sydney dental patients may have been exposed to HIV and hepatitis, according to health authorities.