Mundaring resident Hugh Richardson said he first noticed the makeshift track earlier this year and has since seen the damage escalate, with soil being eroded, uprooted vegetation and torn-down signage prohibiting bike use.

He said as an engineer he was familiar with the signs of artificial constructions on natural trails.

“The trail was constructed by a team of workers who had to move large rocks, cut down small trees and shrubs and dig away the ground in creating jump mounds and skid-slides,” he said.

“Substantial tools and lifting gear or many hands were needed to move slabs of exfoliated granite to bridge the stream and gullies.”

Mr Richardson said the works had caused significant environmental harm.

“Erosion at the second site had definitely been exacerbated after the last set of rainfalls,” he said.

“What used to be a regular garden of spider and donkey orchids is now bare ground and rock with most of the soil and ground cover washed away.

“It was with deep disappointment to see first-hand three mountain cyclists exacerbating the damage of illegal track building through and across Gunjin Gully in the Mundaring National Park.

“The rate of degradation over three weeks since our last visit suggest large swathes of the park will not be available in their natural form for the next generation of recreation bushwalkers.”

He added that some riders admitted to knowing the trail was illegal but continued to use it regardless.

“They said they wouldn’t be deterred – a euphemism for telling me to ‘go away’,” he said.

The land in question sits just south of Mundaring Weir Road near Paull’s Valley Road.

The Department of Biodiversity, Conservation and Attractions (DBCA) confirmed it falls within Beelu National Park and is under its management, not the Shire of Mundaring’s.

In response to questions from Echo News, Shire of Mundaring President Paige McNeil said while the site is not the shire’s responsibility, residents are right to be concerned.

“We all have a responsibility to care for our environment,” she said.

“Damaging or altering natural areas not only harms local ecosystems but also takes away from the shared enjoyment of these spaces by our community.”

Ms McNeil said the shire actively promotes responsible recreation in the Perth Hills through education and collaboration with state agencies.

Mr Richardson said he is not opposed to mountain biking when conducted legally but condemned the “short-term self-gratification” of riders who destroy bushland for personal use.

“Perhaps they just haven’t considered the implications of their vandalism,” he said.

“They have no entitlement to such wasting of a public asset, so it is lost to the community at large.”

Residents who witness vandalism or illegal activity in the area are urged by the shire to report it directly to DBCA.