The Department of Conservation is being forced to spend increasing sums of its $8.5 million Great Walks budget on flying out human waste and maintaining toilets.

Director general Lou Sanson says helicopters fly constant trips from the nine tracks that are attracting record numbers of tourists, 70 per cent of who are from overseas.

"The biggest thing for us is the amount of human waste that we're shifting - the unglamorous side of the huge boom is that we fly everybody's human waste off all camps on the Whanganui River and all the Great Walks," he says.

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The department, which is in line for a boost in funding in this month's budget, is at the very sharp end of the tourism boom and an integral part of shaping perceptions of this country.

Sanson says the department's workers pride themselves at keeping toilets clean as possible.

''People form a different view of a country if it can't keep its toilets clean."

DoC owns 32 per cent of New Zealand and 14 national parks. Last year, of 3.2 million tourists, 1.4 million of them visited the conservation estate.

This number is forecast to almost double to 2.7 million by 2025.

"People are coming [for] magnificent landscapes, sea, rivers - we sit right at the heart of New Zealand's tourism economy and we sit at the heart of New Zealand's brand," Sanson says.

He says this country's reputation as a premier tourist destination will suffer if it doesn't get it right.

"People are buying an experience in New Zealand following some of the best marketing in the world by Tourism New Zealand. They're not coming for endless queues of people on the Tongariro Crossing or they can't get a car park at Franz."

Walkers in the southern crater of the Tongariro Crossing. Photo / Mike Scott
Walkers in the southern crater of the Tongariro Crossing. Photo / Mike Scott

Sanson won't be drawn on what he wants from the budget - being delivered on May 27. The Green Party says DoC funding has been going backwards for the past eight years and wants it restored to 2008 levels.

Last year the bulk of funding - nearly $161 million or 37 per cent - was for the management of natural heritage including the maintenance, restoration and protection of ecosystems, habitats and species. That's mainly spent on eradicating rats, stoats, possums and weeds which are threatening up to 1020 species, according to department figures.

In last year's budget a total of over $143m was allotted for recreational facilities and services, and the management of business concessions (33 per cent of the vote).

''In my view I think we had a fairly favourable hearing and ministers will decide what they want to do in the budget - we have put our issues of growth on the table.''

Sanson has spoken about a hike he did with the United States ambassador last year on the Tongariro Alpine Crossing, a 19.4km one-day trek.

"Every time we stopped we were surrounded by 40 people. That is not my New Zealand. We have got to work this stuff out - these are the real challenges."