Mr Nightingale said young people seemed to be taking them just to get from "A to B" and dumping them when they were done.
"But, of course, some of these are highly valuable items. If anyone sees someone riding a bike and it appears out of place don't hesitate to give police a call.
"It's theft and if they take it from somebody's property it's actually burglary. People need to realise that, they could end up in court."
He said people sometimes stole them and sold them cheap, but the people who bought them could also be held liable.
"It's a mountain biking town, a lot of the staff know how much these bikes are worth and they often stop people on them to ask where they got them from if something looks out of place. Most of the time they don't have a suitable explanation."
He said this was an ongoing problem for Rotorua and asked anyone with information to call Rotorua police on (07) 346 2968 or Crimestoppers anonymously on (0800) 555 111.