Franco,
I don't know about Australia, but in the midwestern United States, there are expansive temperate deciduous forests (comprised of things like oak, maple, hickory, birch, beech, walnut, sumac, etc), on the floors of which rest the leaves from the past several autumns, in various states of decay. It's the "in various states of decay" part which leads to the ground being somewhat squishy due to the layer of leaves. It's not really as squishy as a CCF sleeping pad, imo… but as far as forest floor goes, it's a bit more comfortable to sleep on than the rocky hardpan of a more arid region. In many pine forests, you will see fallen layers of pine needles that function in a similar fashion (thus you can have duff in many different forest types). Pine needles tend to be softer, but the natural layer of needles in a pine forest tends to be a little thinner than the layer of broadleaf leaves in a deciduous forest, so sometimes you need to take a few minutes to rake the needles on the nearby ground into a suitable pile to sleep on.
Not really my cup of tea, sleeping on piles of leaves or pine needles, but in some forest types, it's more viable than others.