The pilot’s blood alcohol concentration (BAC) was found to be 0.081%. However, the examining toxicologist stated that, due to the possibility for post-mortem production of alcohol, that result should be interpreted with caution.
Even with that, it's still quite a concern. And what about Cannabis use? Another
ATSB report dealing specifically with the effects of cannabis use on pilot performance states;
…there were significant effects on pilot performance 24 hours after smoking a single marijuana cigarette. Overall, the pilots demonstrated much more difficulty in aligning with and landing on the runway after 9-THC exposure. There were increases in the number and size of aileron changes, the size of elevator changes and the degree of vertical and lateral deviation from the required flightpath during the approach to land. At 24 hours post-marijuana, the lateral deviation on approach to land was almost twice that of the pre-marijuana test.
Granted the post mortem result on this particular incident was only able to say the inactive metabolite of THC was present suggesting that cannabis was used within the previous two weeks. But it's still interesting to note that even while not experiencing a "high" you can still be effected by the drug.