For Australia, try transfercar.com.au, standbyrelocs.com, or britz.com.au. If you google it, I’m sure there are other options as well. Just be careful, because sometimes it can end up being pretty expensive, even with the special deal. For our car relocation, we paid $5/day for 4 days, but then purchased two extra days for $75 each.
They also gave us $350 towards the passengar ferry from Devonport, Tasmania, to Melbourne, Australia. This covered the cost of the car and part of one person’s ticket. We still had to pay an additional $350 to cover the rest.
The car was diesel, which was good, and all the fuel came up to about $150.
All in all we spent about $600 for the car (fuel, extra days, ferry tickets) for 6 days. Since we slept in the back this included accommodation – we parked at free campgrounds. This was one of our most expensive weeks of our whole trip, but it was well worth it!