Graffiti vandals have attacked a section of the Woolooware Bay Shared Pathway which, ironically, celebrates the environmental riches of the area.
Sutherland Shire Council staff discovered the damage around the Arthur Gietzelt Memorial Lookout at Taren Point on Tuesday and police have been notified.
Removing the graffiti will be costly as specialist contractors have to be called in due to the proximity to mangroves.
The award-winning stage six of the pathway was completed two years ago and is enjoyed by bike riders, joggers and walkers from throughout the shire and far afield.
The lookout faces Towra Point, and signage commemorates the community campaign to stop an airport being built in the 1960s and preserve it as a nature conservation area, including as a sanctuary for endangered seabirds.
Mayor Carmelo Pesce said the shared pathway was "18 years in the making, and to see it disrespected in this way is absolutely disgusting".
"Council contributed $4.2 million to this significant project, to remediate the area and create an ecological sanctuary that could be enjoyed by both endangered wildlife and our community," he said.
Cr Pesce said the community "enjoyed relaxing by the foreshore and viewing the interpretive screens and signage on site".
The project had won five industry awards and was a source of pride to the council and community, he said.
Graffiti vandalism can be reported to the council by using the app Snap, Send Solve that can be downloaded from the council's website.