An ATAR says what you achieved at high school; it is essentially only useful for your first year out of school – it will get you into a uni course or into a job. After that your progress is entirely determined by your performance.
In many ways even more important than your ATAR or “leaving certificate” is your reference. This says who you were at high school: it highlights your habits and character, the things that will determine the progress of the whole of your life. This article from the Sydney Morning Herald puts this in perspective from the point of view of a Vice Chancellor.