Overall, 5100 patients at Carina's Gap Free Smiles have had to be tested, but authorities insist none were infected.

The clinic was temporarily closed for about a month from December 2016, but the complaints were first made in August 2015.

A spokesman for Queensland Health Ombudsman Leon Atkinson-MacEwan confirmed on Thursday that the major health issue was first identified at the Gap Free Smiles Dental Clinic 16 months before the dental clinic was temporarily closed.

"The Health Ombudsman received a health service complaint in August 2015, about a potential risk to the health and safety of the public relating to Dr Mohit Gupta of Brisbane dental clinic, Gap Free Smile," he said in a written statement.

"As reported at the time, the Health Ombudsman responded to that complaint by placing conditions on the practitioner's registration, by commencing an investigation, and by commencing monitoring of the practitioner's compliance with the conditions."

More than 2500 patients presented to the dental clinic between August 2015 and December 23, 2016, based on Queensland Health figures.

The clinic was closed because equipment that should have been used once was being re-used without being sterilised, Queensland's chief health officer Jeanette Young told a public hearing on April 6 this year.

"We thought they were very significant breaches to the point where we then - to the best of our ability - contacted every single patient that had been in the practice since it was first opened in 2014 to alert them to the risk that they had potentially contracted a blood-borne disease," Dr Young told the Health, Communities and Disabilities Services committee.

"Hepatitis C was my major concern. We made up to eight attempts to contact them by phone, we sent out registered letters and asked for all of them to see their treating GP.

"We also set up some clinics (at the Princess Alexandra Hospital) for people to access if they preferred to do it that way. we have managed to contact the majority of the over 5000 patients involved, so it has been an enormous exercise.

"That is how concerned we were at what was going on in that practice."

Some of the clinic's patients were children who had their first trip to the dentist at the clinic.

Queensland Health said there was "no evidence" any patient had contracted hepatitis C, hepatitis B, HIV, or any blood-borne disease and pointed to "typical" figures of notifiable diseases.

The Carina dental clinic has since been inspected and checked by health authorities and is now operating safely.

Information provided to Fairfax Media shows these controls over possible infections remained in place from August 2015 until May 6, 2016.

However, Fairfax Media understands Dr Young was not advised of the health risks until "late" December 2016, at which time she decided to close the clinic.

It became public knowledge when a media statement was issued by Queensland Health.

The dentist, Dr Gupta, was immediately suspended by the Health Ombudsman and was re-instated with conditions in February 2017.

Both Queensland's Office of the Health Ombudsman and the Australian Health Practitioner's Regulation Agency recorded Dr Gupta's temporary suspension in February 2017.

Dr Gupta told Fairfax Media at the clinic on Thursday it was not true that health concerns had been first raised at the dental clinic in August 2015.

"No, it is not true," Dr Gupta said.

However when Dr Gupta was told the Health Ombudsman confirmed complaints were first made in August 2015, he said had nothing further to say and asked Fairfax Media to leave.

Dr Gupta referred any questions about the clinic's operations to the Australian Dental Association's Queensland branch.

ADA Queensland chief executive Ian Meyers said complaints against the clinic's practices were made in two phases, starting in August 2015, and were initially only minor.

Dr Meyers said it was not necessary to close the clinic originally because the complaints were "minor breaches of protocol".

He said "a second issue" was raised in the second round of complaints, but did not know what that complaint was about.

"But that was probably the reason why they shut the clinic down," he said.

Dr Meyer said it was difficult to judge when "minor matters" became serious enough to close down a clinic for checks.

"Ultimately, it is up to the individual practitioner."