I'm going to go out on a limb and assume that the "Mystery Vehicle" XZAR is the Australian equivalent to American Hertz's "Manager's Special" EXAR. If so, the vehicle you get could be nearly anything—you're getting a low price specifically because Hertz isn't committing to any car in particular. It seems to be a way for them to pawn off whatever's leftover, so your expectations should be low, but you might get a nice surprise.

The one and only time I booked a Manager's Special (at PHL), I ended up with a decently equipped Prius—for less than the cost of a Hyundai Accent. Now I personally don't like the Prius, but I think I got a decent value for the bargain basement price.

There is a thread here where people have listed the mystery vehicles they have received.