Guided walking companies are under investigation by DOC after "irregularities" were picked up on the Great Walk booking system.
Six operators offering treks on the Heaphy, Paparoa and Rakiura tracks were suspected of gaming the DOC hut system by making speculative or incomplete bookings, to sell at a later date.
The DOC compliance team said it has followed up with a small number of commercial operators as part of annual check on bookings.
Currently private operators are only allowed to book up to 50 percent of Great Walk hut space for clients, under a concession agreement.
Walking companies can only book spaces as and when they have a confirmed client.
However, due to the first-come-first-served basis and the huge demand for bookings by the general public, there is a pressure for companies to fill up their concession before spaces are filled.
"If operators are found to be in breach of their concession agreements, they must either update their bookings or have the places cancelled," said Steve Taylor, DOC's director of Heritage and Visitors.
Taylor confirmed that six operators were contacted over suspicious bookings, however the number of bunks on the network this represents was tiny.
The bookings in question was "roughly 0.2 per cent of Great Walk hut and campsite spaces across the coming season".
"Suspicious bookings" does not necessarily mean a hut full of Joe and Joanne Bloggses - but could just be a failure by operators to give address details or other required information for clients.
This year there was huge pressure on the most popular Great Walks. This year the Milford Track booked out in 15 minutes after opening.
The concessions for private walking clients represented just 4 per cent bookings during peak seasons. However there is a very complex system in place for managing for-profit operators.
The most popular tracks such as Milford, Routeburn and Kepler are not bookable by guides.
Abel Tasman - with quarter of a million bed nights per season - is the only great walk which operators can make speculative bookings for clients.
In the remaining Great Walks tour guides are able to book up to 50 per cent of bunks for clients, however, they have to provide details with all bookings to prove they have walkers lined up for the hut space.
"Peak season dates on the Great Walks can book out fast and it's important access is equal for everyone," said Taylor. "Anyone who abuses the system does so to the detriment of the public and the vast majority of concessionaires and agents, who offer an important service on these walks and play by the rules."