Riders claim they've found ge rocks and logs deliberately-placed across sections of track in the reserve, managed by Shellharbour City Council, on a number of occasions in recent weeks.

“There definitely seems to be some sort of sabotage of the walking and mountain bike trails," mountain biker and nearby resident Luke Perry-Gore told the Mercury.

“It seems to be mostly moving rocks, kind of double-head size I guess, as a reference, and logs and large branches across the track."

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The 30-year-old regularly rides the trails and walks them with his dogs.

He said the "sabotage" had been happening for about the past month - two or three times a week, sometimes more.

“Occasionally you would see it but it’s been happening very frequently recently," he said.

Mr Perry-Gore believed it was highly unlikely the branches, logs and rocks had been shifted by the weather.

"They’re things that are already embedded in dirt," he said.

"You can see where they’ve come from, someone’s pried them out of the dirt or pulled them from somewhere in the bush and put them across the tracks.

“It’s definitely something that’s deliberate. I suspect it’s trying to discourage people from mountain biking on the tracks; I don’t know what someone’s motive is, but it definitely seems that way."

The lifting and shifting appeared to happen under the cover of darkness, he said.

Riders haven't seen it as it happened, but said the narrowness of the tracks meant the likelihood of catching anyone in the act was slim.

"You don’t see very far ahead of you. I think if someone were doing this they could feel fairly confident that no one’s coming up on them as they do it," he said.

The narrow nature of the tracks also heightened the dangers posed by the unexpected obstacles, Mr Perry-Gore said.

“They are walking tracks as well, so you know to go a safe speed so you can slow down for anyone walking," he said.

“There definitely is some danger from what’s happening."

One positive amid the concern, riders said, was that the sabotage hasn't been anything more sinister. However, Mr Perry-Gore said he had found star pickets sticking out of the ground in the past.

“I’ve even found fishing hooks down there, and it’s not somewhere people would be walking through with any fishing gear," he said.

"I think, again, that’s trying to deter people with tyres, mountain bikers."

Mountain biking in the reserve is allowed, only motorised bikes are banned.

The council said no reports of dangerous obstacles being placed on tracks had been received via the police.

General manager Carey McIntyre said council staff met regularly with police and rangers frequently patrolled the main areas of the reserve. 

"As can be appreciated, there are numerous paths located throughout the reserve, and it is not practical to attend to all of these locations," Mr McIntyre said.

He said there were several designated tracks located within Blackbutt Reserve, but also a number of paths that have been created by riders "carving" their own tracks.

"Illegal trail bike activity is also a main contributor," the GM said.

Mr Perry-Gore said the trails were also used by students at Warilla High School, who did mountain bike riding for sport.

A Department of Education spokesman said the school had not received any reports of unsafe or unsatisfactory trails.

"Warilla High School is committed to high standards in offering the sport of mountain biking in line with Department of Education and Mountain Bike Australia safety guidelines.

"The lead teacher/supervisor is accredited by Mountain Bike Australia," the spokesman said.

"All students undertake a stringent safety briefing and assessment before they are allowed to participate.

"Two committed members of staff oversee the mountain biking sport and conduct thorough reconnaissance of courses.

"To complement this, in response to rumours parents have also conducted sweeps to ensure courses comply with appropriate safety standards."