As mentioned previously, hearing is more than the physical process of vibrations being perceived inside the structure of the ear. Hearing is also about interpretation of those vibrations, and the how brain filters the vibrations our ears physically perceive. (How the brain filters and makes sense of what we, and in this case the koalas, hears.)
The history of the koala as an arboreal (tree dwelling) animal with little predator threat has influenced their hearing filters and reactions to what they hear. From the heights of the treetops, koalas filter out noises such as cars and other motorised vehicles going past. Roads encroach so much koala habitat that many koalas find themselves sleeping in places where there is almost constant vehicle noise.
Have you ever driven along a busy road and seen a koala fast asleep in a tree right beside that road?
By the same filter, a koala does not really differentiate between the sound of a car, chain saw, mower or bulldozer.
The koala has no way to figure that the bulldozer is going to knock down the tree it is in, when thousands of cars and other machinery have gone right past without incident.
In the same line of reasoning, the koala does not perceive they are in the direct path of the car approaching, when it is down on the road. The car sounds the same as the cars that went right past the tree all day long. The koala does not have the kind of executive thinking to figure that this spot on the ground is the place the cars were.
I have often seen a koala look up and watch me approach on the quad bike however. In nearly all instances the koala was already awake, and actually sees my face, and sees me looking intently into the tree. Any koala that has not become very accustomed to my presence will react to me, rather than the quad bike, and specifically to me looking directly at it.
I have tested this by looking away and riding past the tree, then looping back around and being sure not to directly look at the koala or the tree. The koala will nearly always relax, at least to some degree.
I always obtain a much bigger response if I approach the tree on foot.
An individual koala will have variations to their hearing filter depending on the particular habitat they live in and what is normal there. This means a koala that lives in a forest that is isolated from roads completely may respond differently to machinery noise if it is something they do not normally hear at all.