A spectacular ice retreat at the Franz Josef Glacier has surprised experts.
The glacier has retreated 500m in four years, prompting suggestions of a road up the valley as the ice disappears from view.
Dr Brian Anderson, a Victoria University senior research fellow in glaciology, said the 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 big retreat 90 years later, in 1983, Franz Josef Glacier receded about 3km.
Between 1983 and 2008 it advanced almost 1.5km after heavy snowfalls. But in the past four years it has melted almost 500m.
The retreat began in 2008, and last year the ice thinned by about 70m behind the glacier terminal.
A colleague with a seismometer detected "ice quakes" - the ground shaking from an ice collapse - as a huge cavity formed beneath the glacier, eventually causing its 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 helicopter.
The walk from the car park to the terminal face is now 3km.
Department of Conservation spokeswoman Denise Young said it was taking so long for visitors to reach the glacier that fewer were taking guided tours.
The department was considering countering this by building a formed road to allow some vehicles to drive from the car park to the face of the Franz Josef.
It was considering a similar road to the face of its sister glacier, the 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 have to be changed so the road can be built. Changing the plan requires public notification and will take at least eight months.
DoC is also considering reviewing the limit on the number of heli-hikes allowed on the glacier.
Dr Anderson said that although it had been a very cold winter, 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 present debris-covered terminus.
"In general, we expect that the glaciers will get a lot smaller in the coming century, as the climate warms," Dr Anderson said.
- Greymouth StarBy Laura Mills