The Department of Conservation has caught two overseas guiding groups conducting illegal walking tours in Abel Tasman National Park.
A German group with 15 walkers and an English guide with six people have been found operating without a concession in the park at the top of the South Island.
DOC says it is clamping down on illegal commercial activity because it needs to control tourist numbers in the small park.
It says the guided walkers were innocent victims and were allowed to complete their trip.
However, the guiding companies are being asked to explain themselves and face fines of up to $80,000.