A 1-tonne hut has been airlifted in by the NZ Defence Force to help the Department of Conservation protect the world's clearest lake.

It is the clearest lake in the world, but surging visitor numbers to the pristine waters of Blue Lake may be putting that title in jeopardy.

Some visitors have started washing their dishes and bathing in the Nelson Lakes National Park attraction, while others are leaving toilet waste close to the water.

The Department of Conservation (DOC) has decided enough is enough, and has called in the big guns to help.

ALDEN WILLIAMS/STUFF Blue Lake, also known as Rotomairewhenua, had the clearest natural fresh water in the world and was considered sacred by local iwi.

A 1-tonne hut has been airlifted in by the New Zealand Defence Force (NZDF) to house DOC wardens who will protect the lake.

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An NH90 helicopter, manned by Royal New Zealand Air Force and New Zealand Army personnel, moved the shelter about 32 kilometres from St Arnaud to its new location beside Blue Lake.

NH90 captain flight Lieutenant Christopher Ross said the NZDF was proud to assist in protecting the nation's environmental assets.

"This task is the New Zealand Defence Force's latest tangible support for the New Zealand public and our environment," Ross said.

"We are pleased that we are able to use our skills and our aircraft to protect such a pristine environment and a source of pride for New Zealand."

NZDF The New Zealand Defence Force moves a 1-tonne shelter to Blue Lake in the Nelson Lakes National Park using a NH90 helicopter.

Blue Lake, also known as Rotomairewhenua, has the clearest natural fresh water in the world and was considered sacred by local iwi.

DOC Nelson Lakes senior ranger Phil Crawford said visitor numbers to Blue Lake had increased significantly in the past two years as word spread about the clarity of the water.

The country-long tramping track, Te Araroa, passed through the area, and the trail's growing popularity had also played a part in the increase of tourists, Crawford said.

NZDF The hut will house Department of Conservation (DOC) wardens tasked with protecting the lake.

"This is putting pressure on the environment, with visitors leaving toilet waste in the nearby tussock and washing dishes or bathing in the lake," he said.

"Wardens at Blue Lake Hut will enable us to manage high numbers of visitors by staying overnight in the area and ensuring that the lake and environment are protected."

DOC had a history of working with the NZDF on airlift tasks as well as engaging new recruits to work on tracks, Crawford said

NZDF Soldiers unload from the Royal New Zealand Air Force NH90 helicopter.

"We value our relationship with the New Zealand Defence Force and appreciate their logistical expertise," he said.

The shelter would provide accommodation for volunteer wardens at Blue Lake Hut, 1200 metres above sea level.

The Royal New Zealand Air Force's No. 3 Squadron helped in the mission as they were already in the region for Exercise Blackbird, practising flying in mountainous terrain should it be required for search and rescue operations.