The 500m retreat in just four years has been accompanied by 'ice quakes', and given rise to suggestions of pushing a road closer up the valley as the ice slowly disappears from view.
University of Victoria senior research fellow in glaciology, Dr Brian Anderson, said the current retreat was "really unusual and quite amazing".
"While the glacier has always been dramatic in its advances and retreats, the rapidity of the present retreat is remarkable," he said.
Between 1893 and the end of its last major retreat 90 years later, in 1983, Franz Josef Glacier receded a total of about 3km. Between 1983 and 2008 it advanced almost 1.5km after heavy snowfalls in the neve. But in the past four years alone it has melted almost 500m.
The current retreat began in 2008, and last year it thinned the thickness of the ice by about 70m behind the glacier terminal.
A colleague with a seismometer then detected 'ice quakes' - the ground shaking from an ice collapse - as a huge cavity formed beneath the glacier, eventually causing the glacier surface to sink into it.
By January this year a hole had formed in the glacier, putting an end to guided walks.
Tourists are now flown on to the ice by a short helicopter ride.
It is currently a 3km walk from the road end and car park to the terminal face.
Department of Conservation spokeswoman Denise Young said it was taking longer for visitors to reach the glacier, which resulted in a decline in the numbers taking guided tours so the department was currently considering building a formed road to allow some vehicles to drive from the car parks to the terminal faces of both the Franz Josef and its sister glacier, Fox.
Last year about 330,000 people visited Franz Josef, and 184,000 went to Fox.
The Westland National Park plan and park bylaws may need to be changed to allow the road to be built. Changing the plan requires public notification and will take at least eight months.
DOC is also considering reviewing the current limit on the number of heli-hikes allowed on the glacier.
Dr Anderson said although it had been a very cold winter so far, the past few years had been warmer and the glacier had been losing a lot of ice.
It would take more than a year of good snowfalls to make up for the loss.
The ice had continued to collapse into the hole, which was getting "bigger and bigger" and would eventually form the new terminus, about 500m further back from the current debris-covered terminus.
"In general, we expect that the glaciers will get a lot smaller in the coming century, as the climate warms," he said.
It was hard to say exactly how much the glacier had retreated in the past four years, as it was doing so unevenly. The part furthest down the valley had receded by only about 50m since 2008 because it was protected by the insulating debris cover, but along the Waiho River it had retreated out of view by more like 400m in that time, he said.
- Laura Mills of the Greymouth Star
