It's the clearest lake in the world right in Kiwis' backyard, but there is concern it could be at risk if an algae spreads to it.

Rotomairewhenua, or Blue Lake, is nestled deep within the Nelson Lakes National Park. It's not very accessible as it's at least a four-day round trip walk.

But once you arrive the view of the lake is so clear and so blue it almost does not look real. Visibility is between 70-80m, making it incredibly close to optically pure water.

Department of Conservation biodiversity ranger Janet Newell said Blue Lake is striking when you first see it.

"The first impact of Blue Lake is just amazing. It's just so clear and in the right conditions it is a real blue colour and you look at it in other conditions and it's these beautiful greens."

1News caught a ride in a helicopter with DOC as they undertook track maintenance.

Newell said many people don't realise how isolated the area is.

"It's not unusual for people to turn up at the visitor centre and ask where the turnoff for the road is to Blue Lake. They look at a map and see the walking trail as the road."

The lake has grown in popularity in recent years, with people coming from all around the world.

The focus now is ensuring it stays the way it is. An algae that causes a substance called lake snow has been found at nearby lakes.

Cawthron Institute freshwater scientist Susie Wood said caution is needed.

"So we absolutely don't want to get lake snow in Blue Lake. It would change the community, the organisms that live in Blue Lake and also effect that amazing water quality and clarity."

Anyone travelling to the lake is reminded not to touch the lake, or put anything into it.

Someone who knows just how special the place is, and wants it to remain that way, is former NIWA hydrologist Rob Merrilees.

Merrilees first saw Blue Lake in 2001 but sat on the secret for a few years before bringing it to the attention of his colleagues.

He was part of a team that went back and measured the lake's clarity more than a decade ago.

"The first time we went up we just took a little black clipboard, taped it onto a piece of stick and then had a periscope with us and then put it across the edge. I've got a photograph of us doing it just to give an idea of how good it was."

Merrilees would not say he discovered Blue Lake, but he immediately knew it was special because of his work researching Te Waikoropupu Springs in Golden Bay.

"Blue Lake's been there for years and years and lots of people have been through there. So I just happened to be the one who went up there and had the knowledge of the clarity."