An environmental scientist cycling on the track found the material over the Easter long weekend and alerted the venue on Tuesday morning.

The Economic Development Directorate's venue and events services general manager Neale Guthrie said 30 to 40 broken pieces of bonded sheet asbestos, each about the size of a 50¢ piece, were located on the track on land owned by the Australian National University, which co-ordinated a clean-up on Thursday.

Mr Guthrie was only speculating but believed the asbestos could have been left over from an ANU building destroyed during the 2003 bushfires, with the material being missed during the clear-up.

''We don't believe this has been sitting there for five or six years and people have been just riding over it,'' he said. ''I'm pretty confident this stuff has been under the ground and just made it's way to the top. We do visual inspections of all of the trails every two months. And while the guys who do the inspections are not experts looking for asbestos, definitely my venue manager has a reasonable understanding of asbestos, and he can't recall seeing it before.''

The section of the main trunk trail between Red Rock and Tall Trees has been closed and warning signs installed. The trail will remain closed until it is professionally certified as being clear of any asbestos containing material, likely mid-next week.

Mr Guthrie said the affected trail had not been used during the National Mountain Bike Championships in February or the Solo 24-Hour Championships in March.

Mr Guthrie said the government would follow-up with a visual inspection of all the cycling trails. ''I'm always concerned about it. We'll wait and see what the results say. If the results say there's nothing in the air, nothing in the soil, then I'm pretty confident these things have just been exposed in the past few weeks or months through weather,'' he said.

The government issued a statement on Wednesday night to inform the public of the trail closure.

Mr Guthrie said the scientist who found the material had left a message on Tuesday morning, his call was returned on Tuesday afternoon and an on-site meeting was arranged for Wednesday morning. The trail had been closed by Wednesday afternoon. ''I think we moved reasonably fast,'' he said.

Canberra Off-Road Cyclists president Sarah O'Callaghan said members were initially concerned about the risk but were reassured that the asbestos found was bonded. ''Certainly members of the CORC committee did express some concern last night in the bigger picture, that over the years we as CORC spent a lot of time out there building trails and things like that, and were volunteers exposed? We've run big events out there - have people been exposed?'' she said.

''Certainly, the verbal indications are that's not the case, because it is bonded sheet asbestos and it's a couple of very small sections [of the trail].''