I just found out that there can be a small penalty for renting one-way. At certain locations (e.g., Brisbane downtown) you won't be able to get the lowest rate, i.e., the economy car, if you book a one-way rental. If you return the car at the same location, it seems to be about AUD 20/week cheaper, as it lets you book into the economy category (which, AFAIK, offers the same vehicle as the compact cat., not sure why... perhaps with the economy you get a standard, and with the compact you get automatic).

The difference is so small that it's hardly noticeable, however.