Wollongong City Council has asked the community to stay out of the closed section of Mt Keira Road after council staff saw people accessing the site illegally.
The narrow roadway has been plagued by rock falls and shifting soils, prompting several closures to allow emergency repairs.
The council was forced to shut off community access indefinitely in December after geotechnical specialists ruled the state of the road posed too much of a risk to public safety.
Despite the warnings and road blockade, the council said some of its officers had repeatedly seen people accessing the site on mountain bikes.
The council also said there was fresh graffiti on parts of the rock face beside the closed section.
It has not been suggested the bike riders were responsible for the vandalism.
The council's acting infrastructure manager Michael Malone said he was astounded people were ignoring the "road closed" signs.
"It's closed for a reason, and that reason is it's not safe," he said, pointing to the risk of rock falls, landslides and shifts in the camber of the road.
"People using this area are putting themselves at risk of serious injury or even death.
"We know it's popular with walkers and all types of cyclists ... but until we have an understanding that we can make it safe - stay out."
The council's geotechnical officer, Peter Tobin, said Mt Keira Road was "slowly slipping away" in some parts and would require more work to make it safe.
He said soil that had been saturated from natural springs was destabilising the roadway.
The council has spent $1.75 million in the past four years on propping up the fragile roadway.
The Mercury understands geotechnical specialists are reviewing the state of the road and will prepare a report for the council outlining expected remediation works and their associated costs.
The road closure extends from the archery range to the scout camp, while a section between the scout camp and Queen Elizabeth Drive is open to residents and businesses only.