Outside the clinic, Pukekohe mum Maro Hiku told the Herald she was concerned and "scared" for the health of her 6-year-old daughter Remedy.
She said she learned of the health scare on Facebook last night, and was "shocked" to be alerted via social media rather than receive a phone call from a health professional.
Hiku and her daughter, who was treated in September, were told to return to the clinic after making an appointment.
An investigation is under way into the exact nature of the problem and how it happened.
Jordan Sherwin, who took her 6-year-old son Dayton to get tested today, said parents had arrived in droves by mid-morning seeking emergency screening.
"Seats are all full and queues are out the door at the dental clinic at Pukekohe Intermediate with parents checking if they need to bring their child in for screening," she said.
"There's mothers crying in the hallway while their kids are crying getting their bloods taken."
She said it was very traumatic with a lot of upset parents and children.
Within the first few hours about 25 people at any one time were waiting for screening.
A security guard had also been stationed at the clinic.
Two specialist blood-testing clinics opened at 9am for children who had been treated at the Pukekohe dental clinic over the past five months. They will be open 12 hours a day, every day. First results will be available in two days.
Sherwin said she was furious that her son was put at risk.
"How could this go unnoticed for four months? Who is at fault? Why did I have to find out via social media? Absolutely disgusted with Waitemata DHB and ARDS [Auckland Regional Dental Service]!"
Another parent said the health scare had left parents feeling helpless and upset.
"One parent I know is crying her eyes out worrying over it," a mum wrote in an email to the Herald.
"I have children who visited through this period and now face a night worrying about it," she wrote.
"This may sound dramatic but when you read this and know your child/children are part of it, I think any parent would more than understand our being upset right now."
Many parents were critical it had taken more than a week for the health service to notify the public about the contamination.
This morning the health board's chief medical officer Dr Gloria Johnson said the families of every child who was treated at the clinic would be contacted in the coming days.
"We will be aiming to contact every child and family individually within the next seven days and they will all be offered an individual face-to-face appointment and we're aiming to have that done within the next three weeks," said Johnson.
She said the health authorities understood the widespread concern but the risk of infection was tiny.
"We understand completely their concern. We certainly understand though that it's going to be a very anxious time for parents. We do want to reassure them that the risks really are extremely low."
She said parents could call Healthline at any time during the day as well as the health board call centre. Both were briefed about the situation.
Johnson said it was a major health issue because of the large number of children involved.
Families would get results within 48 hours.
The scare involves children who have had a procedure involving compressed air, a drill, extraction or suction.
Staff at Pukekohe Intermediate School say they were only told about a medical health scare affecting their students yesterday afternoon.
A health nurse briefed staff and assured them parents of at-risk children would be notified today.
Year seven pupils are returning to school today.
The dental service and Counties Manukau Health are encouraging distressed families to contact Healthline on 0800 611 116 or talk to their GP if they require further information or support.
