As per above posts, you'll need an average of 75+ to be competitive (low 80's or above and you're close to a sure thing). It'll take some level of dedication, but is definitely doable, and with considerably less effort than for a 90+ ATAR imo. First year subjects mostly aren't all that hard, although some people struggle with the change in learning style, or continue in a haze of post-year 12 partying before having to cram to pass exams (which is fine, but it's a lost opportunity for easier HD's). Make sure you're taking the subjects you can do best in during that first year (e.g. if you excel at written work, do marketing/management; if you're stronger with numbers/quantitative work make sure you do stats).