Shattered workers at Whakapapa skifield on Mt Ruapehu are determined the season will open on time this year after devastating overnight fires.

The suspicious fires destroyed the main chalet at the National Park skifield and an implement shed housing three expensive snow groomers.

Both the shed and the Knoll Ridge Chalet were destroyed and damage could be worth well over $2 million.

Ruapehu Alpine Lifts general manager Dave Mazey said the two buildings were about 200 metres apart and the fires were probably the work of an arsonist.

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The Knoll Ridge Chalet was one of the main service buildings on the Whakapapa ski slopes and housed a cafe, shops, toilets and other services for skiers.

"Burnt to the ground is probably a simple way to put it. There's a concrete slab left with predominantly buckled, bent and burnt steel beams," a shattered Mr Mazey told NZPA today.

The implement shed was also burnt to the ground and the three groomers it housed were destroyed.

The terminal building of the Waterfall Express chairlift, which was about 10 metres from the destroyed chalet building, was also damaged by the fire but Mr Mazey said he was confident the chairlift mechanism would not be affected and the chair would be operating when the season opened.

Access to the destroyed buildings was by chairlift only and firefighters alerted by someone who saw the fire on the mountain could not get to the blaze.

Today a helicopter with a monsoon bucket dumped water on the buildings which were still burning.

He said the fire damage would be "well in excess of $2 million.

"The groomers have a replacement cost of about $500,000 each and buildings of that altitude ... construction costs would be well in excess of $3-4000 a square metre.

"The only access is by helicopter for building materials, supplies and construction equipment," Mr Mazey said.

He said the fire would not stop the skifield opening.

"The chalet facility we have available at that altitude will be a wee bit different from what we had last year."

He said a temporary shelter building may be erected for the season but that would be decided after talks with the insurance company and builders.

Mr Mazey said it would be almost impossible for the fire to spread from one building to the other because they were so far apart.

"There was no wind last night and it was relatively calm and it would be very hard to accept that the fire jumped across that 200 metres of distance."

No one was in either building, which were a 90-minute walk from the road and had no vehicle access, Mr Mazey said.

Police and fire safety officers were investigating the blaze and police said anyone who saw any suspicious activity overnight should call them.

- NZPA