Wellington  - The Tasman Glacier - New Zealand's longest - is retreating at the rate of more than 500 metres a year because of global warming, a scientist said Thursday.

Martin Brook said that the glacier, which sweeps past Mount Cook, the country's highest peak, in the Southern Alps, was melting into a lake that began to form only 35 years ago and was now 7 kilometres long and two kilometres wide.

He told Radio New Zealand that the lake was likely to more than double in length to 16 kilometres over the next 20 years.

"It's just too warm for a glacier to be sustained at such low altitude, 730 metres above sea level, so it melts rapidly," said Brook, a glaciologist at Massey University.

The Tasman Glacier, once 29 kilometres long, was reported last year to have shrunk to 23 kilometres.

It has long been one of New Zealand's biggest attractions for tourists, who take bus trips and scenic flights by helicopter and ski-planes from the Mount Cook village to view the glacier. (dpa)