To practice psychology, you need to be registered with the Psychologists' Registration Board. This requires you to complete an approved 4-year psychology degree (e.g. B Psych or B Sc with Honours in psych) and then either complete a postgrad qualification or do two years of supervised placement.
In your case, the problem is that your GPA is a below the required credit average (or GPA 5+) for Honours (at USyd, at least). How did you go in your psych subjects? You may be able to get in if you have good psych results (i.e. D/HDs) but your overall average was dragged down by other subjects. Alternatively, you may like to think about some other unis that might have lower entry criteria into Honours in psych.