Aoraki could be coming or going

An Otago surveyor whose research knocked 30m of the official height of Aoraki/Mt Cook last year says this week's large rock avalanche on New Zealand's tallest peak does not mean the mountain is continuing to shrink.

The avalanche from the south face of Mt Cook was reported by a pilot on Tuesday.

It has altered the prominent Hillary Ridge and moved the Department of Conservation's Gardiner Hut - which sits above the Hooker Glacier - off its foundations.

Otago National School of Surveying senior lecturer Pascal Sirguey said the slip was a ''normal process of formation of mountains'' and another example of how ''dynamic'' the Southern Alps were.

''They keep evolving ... they are rising by about 1cm per year, but at the same time they experience very strong erosion ... it's a constant process of creation and destruction.''

The erosion could be triggered by factors including heavy rain, earthquakes or ice expanding inside the rock and causing destabilisation.

Dr Sirguey was the leader of a research project which showed Mt Cook was only 3724m above sea level, not 3754m as measured in 1991 after a major rock and ice fall took 10m off the summit.

The data was obtained during an Otago-led climbing expedition last November using a high-accuracy GPS device.

The discrepancy between the old and new heights was because of a 20-year erosion and reshaping process of the ice cap as it adapted to the underlying rock summit.

It was ''hard to say'' whether Mt Cook would reduce in height or go higher in the long term, Dr Sirguey said.

''I don't think we've got the data to say that it is experiencing any trend, really. It could be in a relatively stable position. It could be that in a long time it may regain a lot of the height that we have lost in the recent event. But it's still very small to the actual life of the mountain range.''

Doc media adviser Lizzy Sutcliffe said GNS Science was trying to determine when the avalanche took place, although people in the Mount Cook Village had reported hearing ''loud rumblings'' around 10pm on Monday.

The topography of the Hillary Ridge would be ''quite different'' as a result of the slip, Miss Sutcliffe said.

Doc had no records of visitors to the area in the mountain's intention system at the time and aerial investigations indicated no-one was caught in the incident.

The area affected was a remote location used only by experienced alpine climbers.

Doc was working with GNS Science yesterday to assess the stability of the area in order to carry out a more detailed investigation of the rockfall and the damage.

''Until that assessment is complete, we are asking that people keep out of the Noeline Glacier region of the upper Hooker valley,'' services manager Mike Davies said. In 2011, the South Ridge of Mt Cook was renamed Hillary Ridge in honour of the late Sir Edmund Hillary. Sir Ed was a member of the first party to climb the ridge in 1948.lucy.ibbotson@odt.co.nz