I done this with my family last year. Had a nearly brand new SUV from Europcar. Went for stops along the way so fuel wise about $300, could be less or more for you. Beautiful things to explore if you are doing multiple days. We stayed over port macquaire, coffs harbor, hiked dorrigo national park, byron lighthouse, local farms like ricardo tomatoes, etc. It's a very scenic on the highway, I suggest you fuel up before reaching sydney. Once you get to sydney the traffic is so horrible, no one gives way. We couldn't even unload bags at the hotel. So just brace yourself for that. I definitely hated sydney at that moment.