Rotorua's Department of Conservation branch says it will have to spend thousands of dollars to remove about a dozen wrecked cars and secure and repair a scenic reserve.
DoC Rotorua acting senior ranger Jo Mendonca said a ranger recently found about eight wrecked cars and evidence of people driving their cars into the Patetere Scenic Reserve on Cecil Rd near Mamaku and doing "donuts" and illegally taking firewood from the area.
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Miss Mendonca said the cars had been moved from within the reserve so they could be taken away, but since then people had dumped about another four vehicles next to them.
She said the cost to remove the cars, the laying of new roading metal and new signage for the reserve would run into the thousands of dollars.
In addition, DoC have had to lock the gate to the reserve entrance to try to stop people damaging it.
"It's a shame we have to spend taxpayer money on this sort of thing when we could be using that money to save our birds and forests."
She said the car wrecks were originally spotted a few months ago by a DoC ranger and "we've now found the lawn area badly damaged and have had to lock the gate".
"In the first place, this shouldn't be happening, but we are not sure if it's going to stop.
"We have no idea who is doing this. But, we suspect some of them have been stolen and taken for joyrides and then dumped there.
"There's no excuse dumping them on public reserve land and we want people to know this is what we sometimes have to deal with.
"It's a real shame people are abusing and destroying the reserve and dumping rubbish," she said.