Health experts have warned some people could test positive to HIV or hepatitis after a safety scare at four Sydney dental clinics, but the dentists might not be to blame.

Some of the 12,000 patients affected by the scare are starting to get blood tests back this week after being exposed to potentially contaminated instruments over the course of more than two decades.

Experts say the rate of HIV and hepatitis in the community and the scale of patients affected mean it is possible some people could test positive, without the dentists necessarily being to blame.

ABC analysis suggests the bill to taxpayers because of the bungle could be more than $1 million.

Most patients are going to their GP for a bulk-billed referral for a blood test.

The Level B GP consultation plus three blood tests add up to at least $90 a patient, all billed to Medicare.

NSW Health said 11,251 patients who had an invasive procedure at The Gentle Dentists' surgeries in Campsie and the Sydney Central Business District from 2005 to date should be tested for HIV, Hepatitis B and C.

About 800 patients of Dr Robert Starkenburg Bondi Junction and Surry Hills clinics dating back to 1990 have also been affected.

They were sent letters last week advising them to get tested for the blood-borne diseases after a Dental Council of NSW and NSW Health investigation of the clinics identified multiple infection control breaches.

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Dr Jeremy McAnulty from NSW Health said it was possible some people would test positive for HIV or Hepatitis B or C because of background rates in their community.

"It is therefore expected that a number of patients who attended these clinics will have these infections unrelated to the clinics and it will be difficult to determine the source of infection," she said.

"While the risk was assessed to be low ... NSW Health had a duty of care to advise the general public of the potential risk and the importance of having a blood test."

Patients outraged by delay

Davie Euan MacDonald was a patient of Dr Robert Starkenburg for 15 years.

Davie Euan MacDonald was a patient of Dr Robert Starkenburg for 15 years.

He was shocked when he received the letter in the mail asking him to get a blood test for HIV and Hepatitis B and C.

"I thought to myself, 'What will be, will be'," he said.

But he is angry it took health authorities six months to bring their concerns to the public.

"It was six months after one of the dentists, my dentist, was suspended from practising," he said.

"I think that's outrageous, so something in the system has broken down, clearly."

Authorities have said it took some time to get current addresses for affected people.

Mr MacDonald went to his GP and had the blood tests for HIV and Hepatitis B and C.

"I had to wait six days for the results and I was irritated it took so long but it came back all clear," he said.

Risk to patients low: Australia Dental Association

Australian Dental Association president Dr Rick Olive said patients could feel safe Australia had the highest standards of infection control in the world.

"I think if some tests come back positive for HIV and hepatitis, it's much more likely that the infection was picked up from another source or already present," he said.

He said the dentists were isolated cases and not indicative of wider failures of infection control in Australian dental clinics.

"I don't believe there is a wider problem with infection control in NSW and other states and territories. We have a high level of compliance and the best infection control measures in the world," Dr Rick Olive said.

Affected patients have been told they have a "low" risk of infection but have expressed frustration over what "low" risk means.

University of Sydney oral health expert Mark Schifter says there is only one documented case of accidental patient-to-patient transmission in a dentist's chair.

Oral health expert Mark Schifter from the University of Sydney said there was only one documented case of accidental patient-to-patient transmission in a dentist's chair in the world.

That case occurred in the US where a patient contracted Hepatitis B, which was considered more infectious than HIV or Hepatitis C.

That case baffled experts because the dentist had good infection control procedures.

Other reported cases internationally had been deliberate infections, Associate Professor Schifter said.

However the location of the affected dentists in the inner-city of Sydney did, to a small extent, elevate risk.

"There are certain areas of Sydney where the risk of coming into contact with HIV and acquiring HIV is higher," he said.

"The fact is in certain areas of Sydney particularly among certain sub-populations — that is men who have sex with men — remain highest risk for acquiring HIV infection."

'Compliance and compliance testing' the big concern

NSW Health has told patients they need to be tested because of "infection control breaches".

Professor Schifter said cleaning dental instruments was a four-stage process that included washing, disinfection, sterilisation in a special steam pressure machine followed by packaging.

Breaches could range from failing to stick barcodes on packaging for tracking purposes to failing one of the four cleaning steps.

"The big concern here is not regulation, it's compliance and compliance testing," he said.

"It is from breaches like this we learn what the significance is of infection risk in the dental setting we can therefore revise our procedures going forward into the future."

Critics have also suggested NSW Health only sent out warning letters in English, despite many inner-city patients of The Gentle Dentist being from an Asian background.

One former patient suggested some people who could not read the letter might have disregarded it and failed to get tested.

NSW Health said it worked with the NSW Multicultural Health Communication Service to get information translated and out to relevant community media outlets.

Patients who rang the phone number in the letter could also access a translator.