Keeping the thousands of skiers who descend Mt Ruapehu's slopes safe and blissfully ignorant of the carnage of avalanches is a precise science.

Footage of a controlled avalanche on the North Island's highest peak reveals just how powerful, fast, and wide the sea of snow can move down a commercial ski route.

Two "significant" controlled avalanches closed the Tūroa ski field on Thursday and have been captured in aerial footage as they run right to the ski route boundary.

To ensure skiers never witness such scenes in person takes a concerted effort from a team of scientists with years of experience studying the mountains.

Advertisement

Every day, routine avalanche control procedures are conducted before Whakapapa or Tūroa ski fields are cleared to open safely to the public and staff.

Snow safety officers Ryan Leong and Dave Kelly spend their winters studying messages stored in the complex build-up of snow deposits and layers on Whakapapa and Tūroa ski areas.

View of the upper mountain on the Turoa Skifield, Mt Ruapehu at 12.30pm on August 15 showing avalanches in the top left of the ski area after avalanche control. Photo / Supplied
View of the upper mountain on the Turoa Skifield, Mt Ruapehu at 12.30pm on August 15 showing avalanches in the top left of the ski area after avalanche control. Photo / Supplied

Mt Ruapehu general manager of safety Andy Hoyle says it takes a constant effort to survey the slopes for clues each day in anticipation of the next storm.

"Storms bring precipitation in the form of snow, hail, sleet and wind. Each storm leaves its mark in the stratigraphy of the snowpack," Hoyle says.

"Layers build upon layers, a kind of temporary record of time. Oldest at the bottom, youngest on top.

"Occasionally, some layers are weaker than others and can fail when the load of the layers above gets too heavy. Once things start to move, this is known as an avalanche."

A strong 5.5 quake 20km northwest of Milford Sound last week triggered three avalanches near the Homer Tunnel last week.

Mt Ruapehu can get early snow, as seen in this shot taken on May 29, 2018.
Mt Ruapehu can get early snow, as seen in this shot taken on May 29, 2018.

Hoyle says recent controlled avalanches on Tūroa Ski area on Mt Ruapehu, captured in aerial footage, reveal the risk on commercial slopes could be equally real.

Advertisement

"As shown by the destructive force of these avalanches, it's important visitors to the mountain respect any closures put in place," Andy says.

Andy says the team is often asked, 'what if you just left it'?

"We know that if we left these avalanche hazards uncontrolled, they could occur naturally in the right conditions, threatening personnel and infrastructure," Hoyle says.

"When on the mountain, make sure you are aware of any avalanche control work underway but also other warnings put in place surrounding safety on the mountain."

In October last year, adventurer Jo Morgan, wife of businessman and philanthropist Gareth Morgan, miraculously survived after being buried beneath a major avalanche for 30 minutes on the South Island's Mt Hicks.

Two guides died in the avalanche that moved into the Harper Saddle area near Mt Cook/Aoraki.

Check out www.avalanche.net.nz where you can see photos of other natural avalanche cycles in Tongariro National Park.

View of the upper mountain on the Turoa Skifield, Mt Ruapehu at 12.30pm on August 15 after avalanche control.
View of the upper mountain on the Turoa Skifield, Mt Ruapehu at 12.30pm on August 15 after avalanche control.