Hundreds of dollars of carefully laid pest traps have been damaged in a vandalism spree along the Tikitapu (Blue Lake) walking track.
Volunteers from the University of the 3rd Age or U3A maintain 200 traps along a 7km line in the area on behalf of Whakarewarewa Pest Free.
They are still taking stock to find out exactly how many traps have been lost but it is believed to be more than 100.
U3A volunteer Richard Kean told the Rotorua Daily Post the group couldn't figure out why somebody would do such a thing.
"These weren't rat lovers but rats themselves. Most of our traps have been damaged, tossed aside, or stolen and the markers removed."
Kean is one of 40 U3A volunteers, most more than 70 years old. They've been responsible for the Tikitapu (Blue Lake) traps for the past year and a half.
In that time, they have rid the forest of hundreds of pests, Kean said.
"My wife and I go out every week," Kean said. "We check the traps, put new peanut butter in them and throw away the rats.
"We feel we are contributing to the aim of making New Zealand free of introduced land predators."
U3A co-ordinator Peter Wood said the group members felt their work had a purpose.
"People like doing the work. They see purpose in it, and most people are very appreciative of what we do because it's a vulnerable area.
"But it gets really frustrating when a bunch of hoons come in and ruin it all."
Volunteers first discovered some damage on Monday last week.
"Then the reports started coming in from all over," Wood said.
"They took our traps. They pulled traps out, threw them away, virtually right around the lake.
"We're still in recovery mode. We've still got traps to buy."
Whakarewarewa Pest Free representative Anthony Garea said each trap cost about $90.
"This happens once every two to three months at the moment," Garea said.
"The Blue Lake is one of the places where our traps are damaged the most."
Garea said the U3A volunteers had been doing a fantastic job.
"They love it. They're really hard-working and dedicated. They're our biggest group out there."
Whakarewarewa Pest Free and U3A are still taking stock to find out exactly how many traps have been lost.