The Department of Conservation (DoC) is having to recruit more volunteers to cope with a spike of freedom campers littering and defecating on tracks in the South Island.
DoC says it kicks out about eight illegal campers a day as tourists numbers on the Routeburn, Milford and Kepler tracks reach record highs.
Greg Lind from DoC said the climbing number of illegal campers were ruining the "clean, green" experience for others.
"Human waste and incremental damage are things that occur that's putting pressure on the environment," he said.
"Like anywhere, you end up with human waste and toilet waste, and once you get into the alpine regions up near the Harris Saddle [on the Routeburn Track], those environments are really fragile.
"There is only one place you can camp 500 metres off the trail and if there is continuing pressure on that particular place then it will degrade that alpine environment and we'll end up with damaged ecosystems."
DoC figures show the number of tourists on all nine Great Walks has increased by 10 percent a year for the past three years.
The Routeburn and Milford tracks and huts, which cost $54 a night per adult, are fully booked for this season ending on 27 April.
Camping is permitted, but must be 500 metres from the path.
Mr Lind said illegal, or 'freedom' camping, had always been a problem, but this season DoC was catching more people than usual who were trying to do the Kepler, Routeburn or Milford tracks for free.
This week, DoC is trialling six volunteers on the Milford and Routeburn tracks to try to catch out illegal campers.
"They've got limited powers and they can certainly shift people on, but if people are being a bit truculant over matters they can report that back to us and we'll intercept those people as they leave the track or we'll get them at the huts", Mr Lind said.
Under freedom camping regulations, DoC can hand out $300 fines on the spot.
Christine Wallace, from the Fiordland Water Taxi company in Te Anau, said some people tried to cheat the system, but there were thorough checks in place.
"We take fishermen up that end as well and we always check their didymo clean certificates, it's our backyard and we don't want people's experiences or the environment damaged in any way."
Auckland University of Technology professor of tourism Simon Milne said the government needed to tread carefully, as tourism was the country's biggest money maker.
Tourist numbers are expected to increase from nearly 3 million in 2014 to almost 5 million in 2025, and Mr Milne was not suprised that people were flooding the Great Walks.
"They're marketed heavily overseas, there's a great deal of awareness about them overseas, they are a drawcard for our visitors, so we're a victim of our own success.
"We're now seeing a greater number of visitors, some of who might put off by the prices or maybe just know things are booked out and decide to do it as free and independent travellers."
Mr Milne said it was usually small communities or fragile environments that were affected by freedom campers.
"Freedom campers aren't necessarily any better or worse than any other kind of tourist, but they do obviously bring certain particular impacts and they are something that we're going to have to manage in the future."
He said there were several unpopular options that should be looked into, such as capping visitor numbers or taxing tourists.