Committee member Graeme Wines, who discovered the damage on Sunday, said the changes made the trail dangerous for regular users who would not be expecting the alterations.

Mr Wines said it was lucky no one was injured. “It was certainly a case of track modification that could have brought somebody down,” he said.

“After all the work that has gone into getting a permit...for someone to come in and do illegal modifications is disappointing.”

About two weeks ago, following a two-year wait, the club was issued a permit to clear vegetation from the track and working bees would soon be held. The permit does not allow changes to the actual trail.

Club president Brett Easton said he wanted to emphasise that the illegal works were not sanctioned by the mountain bike club. “We’re just making it abundantly clear that it’s not us,” he said. “We’re trying to do the right thing.

“It undoes a lot of the good work we’ve done.” Mr Wines said he would repair the damage on Monday.

Those responsible had used shovels, which they would have had to bring with them, to dig out part of the existing trail and create a jump. 

Mr Wines said the jump was built up by about half a metre with a steep drop on the other side where the dirt had been dug out.

In other areas, which are off the mountain bike club’s main trail loops, timber slats and garden stakes have been used to modify tracks, branches broken off and extra jumps added. He said the unauthorised work would attracted penalties. 

Mr Easton said the club was in the process of informing the city council and the Department of Environment, Land, Water and Planning. 

He said it was not the first time unauthorised changes had happened at Thunder Point, and was a problem other mountain bike clubs across the state also had to deal with.