NSW National Parks and Wildlife Service will monitor the former night soil depot site in the Royal National Park at Bundeena after vandals damaged more than 30 trees.
The damage was discovered by Spring Gully Protection Group member Mark Da Silva this month.
He said the main area of vandalism was next to the former night soil depot and extended about 100 metres up the Jibbon Hill relic cliff dune.
"The trees have been pushed over, with their trunks snapped low down," Mr Da Silva said. "Some angophoras have had their limbs snapped off."
Mr Da Silva said trees around the site had been vandalised before.
"The local community is very concerned about the ongoing systematic vandalism of this land," he said.
"The Jibbon Hill relic cliff dune has significant natural heritage value. It is the largest and most intact relic cliff dune in the Sydney basin. It and the upland swamps found east and west of the dune support several endangered ecological communities.
"I don't believe that the NPWS are minimising the potential impacts from antisocial behaviour on this land [and] are using the . . . [site] as a staging post for air-lifting materials.
"Materials and rubbish are often stored insecurely at this location and end up on campfires that are frequently lit on the clearing."
An Office of Environment and Heritage spokesman confirmed the vandalism and said it was disappointing because the area was only recently transferred to NPWS from Sutherland Shire Council.
"NPWS recognises the high environmental values of the former night soil depot and [it] will be closely monitored to prevent this reckless activity from occurring again. NPWS will also clean up the site, remove rubbish then regenerate the site," he said.
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