Blue Lake in Tasman is the clearest fresh water lake in the world.
Since gaining that title in 2011, the lake has been attracting more and more visitors, which has put a huge strain on the Department of Conservation (DoC), its facilities and the lake itself.
There's a good reason why visitors commit to two days of hard tramping to reach Blue Lake, nestled in the mountains of Nelson Lakes National Park.
"They used distilled water to calibrate it, and they couldn't tell the difference between distilled water and this," says DoC Nelson Lakes operations manager John Wotherspoon.
It's a high-country oasis, with a DoC hut just metres from the pristine water's edge. Not surprisingly, it's becoming a major attraction.
Six years ago NIWA scientists measured Blue Lake as the clearest in the world. Visitor numbers since then have shot up.
Mr Wotherspoon says 52 people turned up to stay one night last summer. That put a huge strain on the infrastructure, with the hut only having 16 beds and a single toilet.
"We have heard stories from our wardens passing through here that the hut can be full, there can be people on the floor and people camping outside all over the place," Mr Wotherspoon says.
The huge number of visitors has also led to some troubling behaviour beside the lake, which has a world-record underwater visibility of 80 metres.
"It's a horror story, but you hear people are camping by the lake, washing out their porridge pots, rinsing out their dirty old sweaty clothes, maybe even using shampoo and soap. It's the clearest lake in the world. Why treat it like that?" Mr Wotherspoon asks.
DoC is finally taking action, firstly banning visitors from camping near the lake. Secondly, the hut's being given a much-needed renovation, which has required the use of a helicopter due to the hut's remote location.
A second toilet has been built by DoC staff, and tonnes of gravel have been flown in to provide foundations for camping platforms away from the lake.
"Ultimately, it'll be another few months yet," Mr Wotherspoon says. "We're going to be building a cooking shelter for the campers as well.
"Just so the area away from the lake will be a really nice place to hang out."
Mr Wotherspoon hopes the new facilities will allow DoC to continue hosting guests while also protecting Blue Lake.