During the climb to cruise altitude, the pilot in command noticed that the co-pilot's head was resting on a side window and that he was not responding. After gaining assistance from cabin crew he found that the pilot had been ill into an airsickness bag. The co-pilot was restrained in his seat and a return to Brisbane was requested. The co-pilot regained and then lost consciousness a number of times during the arrival. Weather conditions were suitable for a visual approach, and an uneventful landing was made. The pilot was examined by a company doctor, and although no obvious reason for his illness was found, a viral infection was suspected. The co-pilot had been restrained in his seat for his own safety during the landing and his head had been allowed to tilt forward. The doctor suggested that crews be warned that in this type of situation, if the sick crew member's head is not held back, there is a possibility of suffocation due to an obstructed airway.