There are so many different scholarships on offer, that you really need to look at the criteria of each specific one. Generally, universities offer two kinds of scholarships (though, if you look at their websites, there might be other types as well): 1) ones for academic merit which are quite difficult to get. For most of these, you'd want in the high 99s, although it depends where you're going. 2) Ones given to people who have a low socio-economic background. For those, you'll need to show that you are economically disadvantaged. Generally, a high ATAR helps, but that isn't necessarily the most important criteria.
Scholarships can be pretty generous, but at an undergraduate level, you're unlikely to get a scholarship with enough money to independently live off - if you're moving out and stuff, you need a job. The exception to this is if you get a scholarship with the defence force, but if you get one of those, you're pretty much locked into a career with the military for 6+ years.
Also, you usually can defer scholarships, but not always.