The NSW Department of Industry says anti-social and dangerous behaviour was among the reasons why access to the Helensburgh tunnel had to be restricted.
A steel fence was erected at the entrance to the disused railway tunnel, which is famous for its glow worm population, on Friday.
A spokeswoman for the NSW Department of Industry (Lands and Water), which is responsible for the site, said the fence was installed to restrict access to the site, particularly at night.
Anti-social and dangerous behaviour in and around the tunnel had been reported, she said.
“The department also received reports of damage to heritage items on the platform, and there are concerns about the impact of light, noise and pollution on the glow worms in the tunnel,” she said.
The spokeswoman said a gate would provide access for emergency services, Landcare volunteers and other community groups.
“It is unfortunate those who respectfully visit the tunnel are being inconvenienced, but this has been necessary to preserve and protect the tunnel,” she said.