A report by the National Institute of Water and Atmospheric Research (NIWA) said the volume of ice in New Zealand's Southern Alps had shrunk almost 11 percent in the past 30 years.

More than 90 percent of this loss was because the 12 largest glaciers in the mountain range were melting due to rising temperatures, NIWA said.

The glaciers have passed a threshold, causing the ice to collapse and creating large lakes at their base, the report said.

"The 12 big glaciers with these pro-glacial lakes have passed a tipping point," said NIWA's principal scientist Jim Salinger.

"It is not yet clear whether the glaciers will disappear completely with future warming, but they are set to shrink further as they adjust to today's climate," he said.

"And it is already clear that they will not return to their earlier lengths without extraordinary cooling of the climate, because the large lakes now block their advance."

Tasman Glacier, the longest in New Zealand, is now about 23 kilometres (14 miles) long. A five-kilometre lake has formed at its base in the past 30 years. In the 1880s, the glacier was 28 kilometres long.

But increased rainfall -- which falls as snow and feeds the higher reaches of the glaciers -- on the western side of the Alps has meant the results for smaller glaciers on that side are mixed, NIWA said.

These ice sheets advanced during most of the 1980s and 1990s amid periods of higher rainfall.

But the best known of these, the Franz Josef glacier, is still much shorter than in 1900 and has retreated about 230 metres (yards) since 2000, despite gains in the past two years, Salinger said.

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