Speaking from a UNSW point of view, in the first year, it is not too difficult to get credit average as long as you try. In terms of chemistry, HSC chemistry will not help you much in university (as well as physics) except HSC biology will help. Having said that, I think if you put some effort you can get credit average throughout your degree. I got Dn average after first session, and i didn't really like chem (Cr), maths (Cr) and psyc (Cr), but I did well in Bio (HD) which pulled my average up.