This is a late reply for the sender but for others. There is actually a bit of bouldering to be done on Rottnest. Its not world class or anything like it, but will give a few problems here and there. Near the barracks on the beach between the barracks and the visitor center there are a few boulders of really sharp sandstone that didn’t break when climbing on it. Some other stuff too on the north side of the island. Take water shoes, and you can climb some of the stuff out of the water. No point killing real climbing shoes for it.