Hey newbiepacker! I COMPLETELY understand your concern. I'm not sure how long you're going to Oz for (or if you even know how long!) but I am going to Europe this summer for 2 months and am "winging it" pretty much. I have a general idea of what direction I'm taking throughout, and hostel reservations for the first 3 nights, but that is about all.
So that being said, I think it is doable. (Then again, I haven't tried it yet and am nervous, but I am nervous in general b/c this is my first time!) I am going to try and get hostel reservations for the next destination about 2-3 days in advance, when I know what I'm doing. But I must say I would've planned a bit more, but this is a last minute trip; I bought the ticket 3 weeks ago and just graduated from college so have been crazy busy, and I leave next week. If you would feel better planning, you certainly have the time, and plans can (usually) be altered!
I don't know about a last minute flight to Oz, but it sounds like its definitely worth a try, if you're patient and not in a hurry to get over there.
Wish I were going..........Good luck!!!!!!!!!!!