Two other male riders and one female passenger sustained serious but non-life threatening injuries in a crash.

The accident occurred on the Monaro Highway five kilometres south of the intersection between the highway and Old Cooma Road, near Royalla.

ACT officer in charge of traffic operations, station sergeant Susan Ball, said the collision had occurred between riders travelling north and south.

She said the incident had been particularly devastating as the family of the rider who was killed had been travelling behind his motorbike when the incident occurred.

"The family were here at the scene and were involved in witnessing the collision, so this is a very traumatic incident," she said.

Station sergeant Ball said there were no cars involved on the long stretch of straight road and it was a clear, sunny day at the time of the crash.

"At this stage we don't think a car or a kangaroo or anything else was involved. We have a number of witnesses we still need to talk to establish exactly what occurred," she said.

Witnesses to the accident had been on the scene since it occurred and were assisting police.

ACT Ambulance transported the three injured people involved to The Canberra Hospital.

The Monaro Highway was closed in both directions for about seven hours following the crash.