My daughter did the same thing with a friend last year, except she did it the other way around starting in Perth and finishing up at Airlie Beach. I persuaded her to buy an old bunky as that way they would be able to see a lot more and go off the beaten track. It also worked out cheaper for them.
Of course, there are some weirdos but generally I'd say it's pretty safe. My greatest concern for them was crossing the Nullarbor with the huge distances involved and no mobile phone coverage. They also came back through the desert which I wouldn't have been very happy about had I known in advance !
Anyway, it all turned out for the best. They had a memorable time and I bought the bunky off them when they got back and am still driving it today as it's so cheap to run.
I'm sure Sophie would be happy to give Beckie the benefit of her experiences