An ACT education directorate spokeswoman said garden beds at Harrison School in Gungahlin were tested on Tuesday, following concerns raised by a parent on Monday.

The school did not write to parents about the issue until Thursday, by which time the government had confirmed that "a small amount" of non-friable asbestos was present in some garden beds.

The scale of the issue remains unclear, with the education directorate unable to say how the asbestos came to be at the school, who brought it in or when it got there. It said an investigation into those questions was underway, and samples would be taken from every garden bed at the school.

The directorate also defended the decision not to cordon off the affected area as soon as concerns were raised, instead waiting until the presence of asbestos had been confirmed.

"As a new school, Harrison School was not known to have any hazardous materials utilised in any construction previously, so it was highly unlikely that there would be asbestos-containing material on site," a directorate spokeswoman said.

"The areas were isolated as quickly as possible following confirmation [it had been] found in recycled building material mixed with coarse gravel as ground cover in the garden beds."

Tests confirmed the discovery on Wednesday but parents who spoke to The Canberra Times questioned why they weren't notified until late on Thursday afternoon, after reporters had arrived at the school gate.

Julie Waters said her daughter, a year 5 student at the school, still hadn't been told what was going on or to stay away from the garden beds on Friday.

"It's so secretive; we should have been told as soon as they got the [test] results," Ms Waters said.

"No one really knows right now what damage has been done. I don't know whether I should send [my daughter] back."

Harrison School's Parents & Community Association president Jackie Floro said while she shared the concern of the school community, she was satisfied the school was doing the best it could to handle the situation with limited resources.

"But communication needs to be improved by the directorate," she said.

WorkSafe ACT commissioner Greg Jones said until the investigation at the school was completed, it would remain unclear whether similar contamination may have occurred at other sites.

He said it was not practical to do pre-emptive testing elsewhere because it could potentially involve thousands of locations.

"We need to identify the timing and the source, then we can figure out where else it may have been sent," Mr Jones said.

He said he was comfortable with the education directorate's decision not to close the school, saying it was made on advice from the licensed asbestos assessor who tested the garden beds.

"I know that the assessor indicated that the bonded material was non-friable and in small quantities, and the risk was minimal," Mr Jones said.

The directorate previously said exposure to airborne asbestos was unlikely unless the non-friable asbestos was disturbed.

A spokeswoman for the ACT's peak public school parents' association said the school had cordoned off more of the school on Friday and appeared to be taking a cautious approach.

"We are extremely concerned as the garden beds may have been very accessible to the students," she said.

"Parents are understandably worried... We would expect that the directorate will want to thoroughly investigate how this could have happened."

In a letter to parents on Thursday, Harrison School principal Jason Holmes said the asbestos would be removed some time next week out of hours.

ACT education minister Yvette Berry was also left on the back foot on Thursday, alerted to the situation just shortly before the Canberra Liberals held a press conference questioning the government's decision to keep the school open.

On Friday, secretary of the ACT branch of the Australian Education Union Glenn Fowler blasted the Liberals for "fear-mongering".

He said he had since spoken to the principal and staff at the Harrison School and had been assured an appropriate and measured response was underway.

But Liberal planning spokesman Mark Parton said he had been contacted by numerous parents concerned the situation would never have been brought to light without the opposition's intervention.