A sting on tour businesses operating without a concession in Milford Sound has found up to a third were operating illegally.

DOC Te Anau operations manager Greg Lind said they have had rangers checking tour buses and guides have concessions to operate in the national park throughout the summer. Lind said the rangers found 37 per cent of tour buses that pulled into the Mirror Lakes car park didn’t have a concession on one day.

“They are seeing unauthorised operators most days,” Lind said. “It’s not mum and dad operators, it’s large-scale businesses.

“We are starting with a soft approach and don’t prosecute. We require them to apply for a concession. But if they don’t apply by next season they face prosecution, so it’s a big deal.”

Under the Conservation Act, any commercial operation on conservation land requires a concession. The maximum penalty is five years imprisonment or a $300,000 fine.

DOC, police, immigration and transport officials have also held a checkpoint on two days over summer, checking the details of every vehicle on Milford Road.

The crackdown is part of DOC’s goal to double income from concessions over the next decade. DOC has employed six extra rangers in Fiordland over summer, as part of a $2 million programme to fund more frontline staff during peak times in Aoraki/Mt Cook, Mt Aspiring, Fiordland and Westland Tai Poutini national parks.

At Mt Aspiring National Park, five extra staff have been employed over the summer.

Senior ranger Annette Grieve said three staff were working on front-country sites, cleaning toilets and monitoring freedom camping, one interpretation ranger was based at popular sites such as Blue Pools, and a full-time ranger was monitoring concession compliance. So far it was investigating 20 non-concessioned operators, mostly tour buses and guided hikes.

Aoraki/Mt Cook operations manager Brent Swanson said it had employed one extra full-time ranger for the summer, who was mostly cleaning toilets and working in the visitor centre.

“That’s been our priority – we haven’t been able to put time into concession compliance,” Swanson said.