WELLINGTON - New Zealand scientists say they have found what might be the world's clearest freshwater body in a national park in the South Island.

Researchers at the National Institute of Water and Atmospheric Research (NIWA) said Monday that the "extreme visual clarity" of Blue Lake was "perhaps only exceeded worldwide by certain ocean waters" such as those in the southeastern Pacific near Easter Island.

The NIWA scientists found over six visits that horizontal visibility in Blue Lake, set in the Nelson Lakes National Park, usually ranged from 70 to 80 meters.

"The theoretical visibility in distilled water is about 80 meters, as estimated from the best available instrumental measurements in the laboratory," NIWA aquatic optics specialist Dr Rob Davies-Colley said in a statement.

"So Blue Lake is a close approach to optically pure water."

Most divers considered a visual range, as in very clear seawater, of at least 40 meters to be "outstanding," said the statement.

NIWA hydrologist Rob Merrilees first recognized Blue Lake might be optically outstanding, after observing how the water body appeared broadly similar to the nearby Te Waikoropupu Springs, in which a visibility of 63 meters had been reported.

Merrilees and Davies-Colley organized a scientific study of the lake with NIWA scientist Mark Gall, an expert in ocean optics instrumentation.

Blue Lake was characterized by blue-violet hues seen only in the very clearest natural waters, said the statement.

It was highly unusual in its geological and hydrological setting, being apparently spring-fed from neighboring glacial Lake Constance.

Almost all suspended particles appeared to be filtered from the water as it passed through landslide debris that formed a dam between the two lakes, which probably accounted for its extreme visual clarity, said the NIWA statement.