It was a still, crisp, bluebird day of cloudless skies. The snow on the steep, intimidating off-piste route down to Zurs from the top of the Valluga above St Anton am Arlberg looked benign, even inviting.

Inviting enough to tempt a group of six German skiers and their guide, an instructor from a nearby village in the Arlberg, to head out on the run. They had ascended to the viewing platform at 2,811-metres in two groups via the tiny five-person cable-car from the main Valluga top station. You can take skis on the final leg of the cable-car journey only if you have a mountain guide with you.

My guide Harald and I, and three others in our group, had also taken the little cable-car to the observation platform. We had not taken our skis…we had no plans to ski to Zurs that day.

We watched as the group assembled, left the platform and stepped into their bindings on the narrow strip of snow at the top of the run. They made the first traverse, directly below us. ‘I wouldn’t be on that slope today,’ said Harald, standing beside me as we both watched the group. ‘In fact, I haven’t done the Zurs run at all this season. By now, the middle of January, I would normally have done it several times – but the conditions have not been right. First not enough snow, then new snow but not on a good base.’

The avalanche risk was three on a scale of five. Catastrophe was minutes away.

The group reached the end of the first traverse. They turned for the second traverse, this time diagonal, losing altitude. Some made kick-turns. But instead of making the traverse one by one with wide intervals they followed each other closely. ‘They’re doing it all wrong,’ said Harald. ‘They shouldn’t be gathered together on that slope.’

I remarked to him: ‘A couple of them don’t look as if they should be doing that run.’

We looked on as the group made two more traverses, some of them again making kick-turns. This brought them to a wider pitch, several hundred metres down the slope from us, and three of them skied out of our line of sight behind a rocky outcrop. Four were still in sight, the two closest to us making tentative turns as they descended.

Further down was the guide, and a little ahead of him another of the group. Suddenly, the skier furthest ahead fell to the ground as if pole-axed. At the same time I saw clouds of snow, like billowing smoke, rise from behind the rocky outcrop.

The three skiers I could see still on their feet had all stopped and were staring to their left. Harald had just left my side to walk to the other side of the viewing platform. ‘Something’s happened,’ I called to him. ‘I think it’s a slide.’ It seemed plain at that moment that the skier on the ground had been knocked flat by the shockwave of an avalanche.

A man standing next to me, a mountain guide as it turned out, was already on his mobile phone, as was Harald. The plumes of billowing snow increased and moved rapidly down the mountainside, over an area where we knew were cliffs.

The skier who appeared to be the guide made his way to the man on the ground, picking up just above him a ski he had lost in his fall. He hadn’t moved since falling. The guide appeared to have a brief conversation with him and laid the ski at his side. He at last moved and seemed to respond to the guide.

The guide left him and skied slowly towards the area where the avalanche had started and went out of sight behind the rocky outcrop.

Three of the German skiers had been swept away by the avalanche, which travelled 800 metres down the mountain. Two of them were swept over cliffs. Both were killed.

The third was not carried so far and survived. It transpired that the two who died were not buried in the avalanche but received their fatal injuries from their plunge over the cliff amid the snow and debris.

Three helicopters were quite soon on the scene, and attempts made in vain to resuscitate the victims. As Harald and the other mountain guide on the viewing platform kept in touch with the emergency services, rescue patrollers appeared by our side. But conditions were so perilous that they sensibly took a while before deciding to risk the descent to join the rescue operation.

The three remaining skiers in the group remained stock still, the fallen man having eventually climbed to his feet, on the steep slope for what seemed an eternity, their stance one of desolation and disbelief.

I have been out with ski patrol checking for avalanche risk and setting off controlled avalanches, and have seen many avalanches from safe distances in remote areas and skied through the debris of some. This is the first time I have witnessed a fatal avalanche and it is a deeply sobering experience – especially with the victims having skied off from beside us just minutes before and myself and my guide feeling a sense of such disquiet as we watched the awful events unfolding.

The sense of shock and sadness is profound – quickly followed by reflection that it has often been a judgment call by myself, guides and fellow skiers on whether or not to be on a potentially vulnerable slope at a particular time. Those of us who love the mountains like to think that our respect for them never wavers and that safety is our top priority. But there will inevitably have been times when enthusiasm has outweighed caution.

The lessons are plain – to constantly reinforce that respect and if there is ever the merest doubt about conditions, to exercise total discretion. The uncertain start to the season has left many slopes with no firm base to form a bond with new snow. Off-piste in many areas has never been more dangerous.  Stay safe. There are plenty of secure slopes on which to have fun.

But they are lessons too late for two fellow skiers who set out that sunny morning with feelings of such excitement and anticipation.