If I were in your situation, I would decide and limit myself to the university I'd most like to go to and then find the cheapest air fare possible. Car hire might not be worth it, public transport might be an option? I'd imagine most of those places would have some kind of service that travels to and from that university.
A train might work out cheaper than an airfare. Checking countrylink, if you get a student ticket (will you still be considered a secondary student?), a train to Sydney from Southern Cross is ~$65. Return journey is the same price. Of course, if you're going there by train, you wouldn't be able to come back on the same day.
Like you said, travelling to all those places would probably be too expensive. Travelling to one state might end up being pretty costly in itself. Other costs such as food, if you're staying there overnight or whatever might rack up as well.