I was in Australia for 2-3 weeks 2 years ago in March. I rent a car and drive from Melbourne (+Great Ocean Road) to Brisbane (+Fraser Island). Its hard to say what outdoor attraction to suggest to you and that are non-touristy. I can say that also known places like Great Ocean Road was not so full of tourists when I was there but of course anyway I was not alone there. :) I liked Fitzroy Falls, Royal National Park near Sydney, also in Blue Mountains you can find a lot of trails without much people, places around Port Stephens are also nice, ... and much more. Australia is so big so you can find a lot of interesting attractions, maybe it is just good to avoid to visit some attractions around some holidays.