If you intend to drop off the vehicle at a different location from the pick up location, then tick the box “Return to a different location”. Just type Melbourne into the search machine and choose Melbourne Airport. Then type Brisbane and choose Brisbane Airport as drop off location. When cars are shown it is possible between these locations.
If no cars are displayed at STEP2, then one way is not permitted between these locations.
For one way rentals sometimes it can be a fee. This is almost always included in the total rental rate and payable at the desk on arrival.