Satelitte images show a small lake has started forming and then disappearing on a regular basis at the junction of the Tasman and Hochstetter Glaciers in recent months.
Scientists conducting the annual Glacier Snowline Survey over the South Island later this month are keeping a watchful eye on the lake.
Niwa principal scientist and survey leader Andrew Lorrey said the lake appeared to have been periodically forming and then disappearing since about October last year.
"We suspect it may build to a certain size and is then siphoned away, possibly underneath the Hochstetter Glacier, into Tasman Lake.
"What we are seeing from preliminary satellite images is a potential hazard forming, that could increase ice calving off the glacier."
The formation of an ephemeral lake on the left flank of the Tasman Glacier terminus was predicted by the late Dr Trevor Chinn, who had been part of the glacier survey since its inception in 1977, Dr Lorrey said.
The Hochstetter and Tasman Glaciers join together upstream of Tasman Lake, but Dr Lorrey said cumulative ice loss would eventually mean they would become two separate entities.
Niwa and Victoria University of Wellington scientists complete an aerial survey of 50 South Island glaciers at the end of every summer, taking thousands of photographs to evaluate the snowline altitude and build 3-D models of the glaciers.
These are then used to assess how much of the previous winter’s snow has remained on each glacier to contribute to long-term glacial ice accumulation.
It provides a valuable long-term record of how New Zealand’s glaciers have retreated over time due to climate change.
In recent years, the effect of marine heatwaves and record temperatures has been harsh.
Dr Lorrey said he expected to see a good blanket of snow over the Southern Alps this year, but believed it would only be masking the true extent of ice loss.
Ice loss has amounted to about 30% since surveying began.
"The snowline may be sitting pretty close to normal at the moment, but the overall health of the glaciers is completely different."
The aerial survey will take place later this month.
