On their home page currently, there's a macromedia flash advert indicating "$20 off QF return flight bookings" (or something like that).
Click the link and you can only book return flights for the $20 discount.
Here's how to do the one-way stuff.
Go back to the home page, select "more search options". Then for one way flights, specifying Qantas, and it will give those as well as the $20 discount.