Fly in MEL, fly out SYD, or vice-versa.

Then arrange your itinerary as a loop - one-way city hopping, that is.

If you can firm up your schedule, the discount flights are the way to go. Unless you absolutely need the flexibility for ticket change, airpass is a whole lot more expensive than the discount fares. Also, some discount fares (not the cheapest one) do allow changes up to 24 hrs before travel, with surcharge.