A lake at the top of New Zealand's South Island has some of the clearest water in the world, a study has found.

Research by the National Institute of Water and Atmospheric Research shows that the visibility in Blue Lake in the Nelson Lakes National Park is up to 80 metres. The water is considered almost as "optically clear" as distilled water.

It surpasses the renowned Te Waikoropupu, or Pupu, Springs in Golden Bay, where visibility of up to 63m has been reported. Water visibility was considered outstanding when it exceeded about 40m, Niwa said.

The lake's clarity might be bettered only by ocean waters such as those in the southeast Pacific near Easter Island.

"Blue Lake is characterised by blue-violet hues seen only in the very clearest natural waters," Niwa said.

"It is highly unusual in its geological and hydrological setting, being apparently spring-fed from neighbouring glacial Lake Constance. Almost all suspended particles appear to be filtered from the water as it passes through landslide debris that forms a dam between the two lakes – which probably accounts for its extreme visual clarity."

Niwa scientist Mark Gall said researchers suspended a black disc, a metre in diameter, under a buoy. They then paddled away in a boat while looking at the disc. Visibility was measured when the disc disappeared.

Niwa aquatic optics specialist Rob Davies-Colley said it was highly significant that the clarity of the water in the lake was close to pure water.

"The absorption of light by pure water is not well known in some parts of the spectrum, including the ultraviolet."

Some of the best data might be collected from Blue Lake, he said.

- © Fairfax NZ News