Mountain bikers and walkers who frequent the popular "Bulli Downhill" tracks have been warned to stay out of State Government-owned land as "extensive" works are undertaken around the bush location.
NSW Roads and Maritime Services (RMS) said its contractors would carry out "extensive land management works" to repair "damage caused by unauthorised recreational users".
Bushwalkers and trail runners use the fire trails regularly, as do occasional motorbike riders. But mountain bikers say the damage to fire trails has been caused by weather - and a lack of maintenance - rather than wheels.
Police have been in the area taking names of riders in recent weeks, and more "high-visibility police patrols" have been threatened in the bush where the mountain bike tracks are located.
In a letter to residents RMS says the "illegal" access by recreational users had caused several problems, including degradation of fire trails and access roads, damage to fences and barriers and destruction of plants.
RMS also warns of the risk of exposure to hazardous materials which are a hangover from the site's previous use as a mine. It's understood these materials are piles of asbestos covered in dirt.
Wollongong mountain bike rider and advocate Jerry Harriman said people had been riding there for perhaps 30 years.
He said riders and walkers weren't to blame damage fire trails.
"Mountain bike riders keep it clear, clear logs from the trails," he said. "It's just degradation from not being maintained. It hasn't been graded for six or seven years. When it rains there's a river going down there chunking out spots."
Mountain bikers have built up the tracks over years, using dirt, logs and sometimes planks to create jumps, drops and "berms" for bush riding.
It appears the work may not destroy the riding trails, but would focus on land between Sandhurst St, Highlands Pde, and the Bulli to Rixons Pass fire trail - tracks used by riders to access the start of the downhill lines.
Transport for NSW said it intends to carry out the works "in the coming months".
Long-time mountain bike rider Keiran Volk said if the area was blocked off, there could be a significant reaction from a wide range of people in the community.
"A lot of people use that area for walking their dogs, walking with their kids, trail running," he said.
"It's a pretty wide user base that uses the area."
Signs have gone up warning people to stay out.
"Anyone accessing the site without approval will put workers and those trespassing at risk of serious harm," RMS warned in its letter.
"Following the remediation of the land it is important that the site avoids further use to ensure the works have time to stabilise against weather events and remain useable by emergency services and Transport for NSW workers."