a subject is almost always 6 credit points. im assuming it says 12 credit points in a year or whatever, so yeah you dont need to do any sciences other than psyc. for psych in first year theres only one subject a semester available.
theres no assumed knowledge for psyc, bio and chem will give you very little advantage. very little maths is done in first year. in second year psyc you need to do two psych subjects a semester (theres only two to choose from, so all of the available subjects) if you want to major in psyc. one of these is a statistics subject, where you do have to do lots of calculations by hand . most people find this subject quite difficult. but there are alot of people doing the subject with much less maths experience than you (people who havent done maths in the hsc, or did general maths or whatever) so i wouldnt worry about it too much.