I am currently doing my honours in psychology at Sydney University as part of a Bachelor of Psychology (Honours). Like the others before me have said, you''ll need an average of at least 75 (Distinction) to make it into honours (but if you're majoring in honours in a BA instead of doing a B. Psychology you may even need a higher average). The only advantage of doing a B. Psych is you are automatically admitted a place in psychology honours as long as you achieve 75. But the rest of the places are fought out between the people who major in psychology.
To transfer from a BA Psych into B.Psych is possible, but very hard. You would need to re-apply through UAC and make a good case for why you should be doing a B. Psych (like a good average at uni and an ATAR that was just not good enough) at the end of 1st year.
If you want to be a registered psychology, you will need to complete your honours and then you will have to do a accredited post-grad degree in clinical psychology (could be a masters or doctorate, as long as its accredited), which typically will take 3 years (+ another year supervision afterwards). The entry into these post-grad clinical courses are much more competitive than the entry into honours. As an example, only one of the 80 people doing psych honours last year ended up doing a Doctor of Clinical Psychology at Usyd. However, clinical psychology is not the only option after finishing honours. You can do forensic or organisational psychology (both allow you to be a registered psychologist) or you may choose to do counselling.
