A man next to the winch operator who launched a glider into the air seconds before it collided with another glider is mystified how the fatal accident happened.

The ground crew including the winch operator and pilots of both gliders were on the same radio network on Saturday afternoon.

A Sydney man, 50, was killed and his 70-year-old co-pilot from Tallong was seriously injured in the accident 20 kilometres north of Goulburn.

The other pilot, a 72-year-old ACT man, landed safely in his single-seater uninjured.

Investigators said later the incoming glider clipped the tail of the two-seater, which then nose-dived to the ground.

Seconds earlier ground crew had called the winch operator to stop the winch, according to the Southern Tablelands Gliding Club's Colin Veal, who was sitting next to the operator.

But it was too late.

''He immediately stopped the winch, which was all he could do.

''It stops virtually instantly. But, of course, the glider is still moving, it doesn't stop instantly.''

The two-seater with a 17.5-metre wingspan hit the ground vertically and was severely damaged.

Mr Veal called 000. Police and ambulance arrived at the airfield about eight kilometres from the Hume Highway, about 15 minutes later. A NSW Ambulance helicopter carried the injured man to Liverpool Hospital.

The single-seater, which has a 15-metre wingspan, had been in the air for about half an hour before landing.

Mr Veal, a former president of the club, which trains glider pilots, said conditions were ''absolutely beautiful''. Only two gliders and six club members were operating from the airfield on Saturday.

Several flights had preceded the tragic one. ''You try and analyse what is going on, but I don't know how it could have happened,'' he said.

Mr Veal and the winch operator were one-and-a-half kilometre from other people who had helped with the launch at the opposite end of the airstrip.

''You can't see why he would take off if he knew they were coming.

''I don't understand how that all happened because you would not expect them to move if they knew someone was coming.''

Drew McKinnie from Gliding Federation of Australia and police said the pilots were experienced.

In September 2007, Peter Moran, 64, of Queanbeyan, died shortly after his glider was winched into the air, banked to the left and crashed to the ground. A friend, Colvin Berry, 68, suffered severe leg injuries.

In July 2008, a 56-year-old Goulburn man injured his leg after his glider clipped power lines as he was landing.

The 40-member club, an amalgamation of several regional clubs, has been flying gliders from ''Lockyersleigh'' for more than 40 years.

Lockyersleigh's owner Matt Onions said the club was professional and its members passionate.

One accident was bad luck, but after two in recent years he was most interested in the investigation's outcome.

The airfield has been closed and a coronial report is being prepared.