The incident poses a significant health risk for the person steering the mower who is likely to have breathed in asbestos fibres.

WorkSafe ACT Commissioner Mark McCabe sent two inspectors to the site after emergency services were notified of the incident at 10:32am.

"From what I understand a contractor was cutting long grass on the edge of a fire trail off Brindabella Road at Coree," Mr McCabe said.

"He ran over what appears to be illegally dumped asbestos. Because it was a mower that he was operating, it caused the material to be broken up and released into the air."

Mr McCabe said the site had been cordoned off. The material was assessed by Robson Environmental and later found to contain asbestos.

"We have engaged licensed asbestos assessors in order to determine how to get rid of the material and the action has been put in place to ensure the safety of the workers involved," Mr McCabe said.

"We're quite concerned at the likely exposure. There is a particularly strong likelihood [the worker] would have breathed some of that in.

"The fact someone inhaled that doesn't mean an automatic disease or condition but it is the way these things do happen."

Mr McCabe said determining where the dumped material came from would be extremely difficult.

"No one would have known it was there or thought to look," he said.

"Unless there's something in the rubble that was left that tells us where it came from, it's likely to be very hard to establish what the source is."

ACT Fire and Rescue also attended the scene on Wednesday morning.