Then there's the Gumtree option. Gumtree is a web site similar to Craigslist where you can post where you would like to go and see if you can't hitch a ride with someone going your way. This option is obviously pretty cheap, maybe you just pitch in for gas, but it really only works if you have the time to wait for the right ride.
Since I was short on cash and on time I opted for the third option, Greyhound Australia. They offer plenty of options to get you from A to B with no frills in between. The coaches are comfortable, booking was easy, and I found the staff and drivers to be very friendly and helpful. I ended up buying a ticket from Darwin to Alice Springs, a 19-hour ride for $134. Then I chose to fly from Alice Springs to Cairns on Quantas for about $390, only because the way the bus schedule worked it would have taken me 4 days to make the trip for the same price versus a 2-hour flight and I needed to save time where I could. The rest of my travel was covered by a Cairns to Melbourne hop-on-hop-off pass, which came to $425 with my YHA discount. So all together my travel costs came to just under $1k. Not too bad I think; I had assumed closer to $2k in my original planning. Another thing I did to save time and money was to plan most of my trips on overnight buses, saving money on accommodation and saving myself the day time to enjoy the places I visited. thus decision led to a few restless nights, but I think it was worth it.
Under the circumstances, I think I made the right decision going with Greyhound and I would highly recommend their service.