The aircraft had been out of service for several months, undergoing engine maintenance. This was the first flight following the completion of that maintenance. After takeoff the aircraft was observed to climb normally to about 1500 feet. It was then observed to begin trailing black smoke and shortly afterwards turned towards the airfield and began to lose height rapidly, until descending out of view behind trees. The aircraft touched down heavily, tail first, in a paddock, bounced and following the second touch down slewed sideways through a fence and down an embankment. An inspection of the engine revealed that both the right and left camshaft inclined drive upper bevel shaft gears had been overheated as a result of a lack of lubrication. The distortion of the left drive was sufficient to disconnect it from its camshaft drive bevel gear resulting in the subsequent loss of power. The engine was fitted with an external oil priming system to lubricate the camshaft drives during and immediately after starting. This system was connected on each side into the appropriate valve train oil supply by "T" fittings and flexible hose assemblies. During reassembly, associated with the latest engine maintenance, the "T" fittings were omitted and the fittings in the front case drillways were blanked with plugs, effectively cutting off the engine oil supply to the camshaft drives. It was evident that the maintenance personnel involved were unaware of the camshaft lubrication system.