Thus far, it's been based on the year completed. Because this leads to people stacking their 3rd year with 1st year units, the universities are currently in the process of revising the GPA calculation process. A lot of people seem to think that they'll be altering it to level of subject rather than year of study but nobody knows for sure.
And genetics health society is an interesting subject (I loved it). The first 3 weeks are basic genetics (plus some new, advanced stuff simplified) and will be easy for science students/tough for arts students. The rest of the subject is legal/ethical etc. perspectives on genetics. There's even a "genetics in art" lecture sequence that is universally ignored by the students haha.
edit,
i would actually keep breadth options open. don't do them all this year, because I can guarantee you that doing 4 science subjects in 2nd/3rd year is painful. In 2nd year I took creative writing, which isn't particularly study intensive during semester - I got to focus on my 2 subjects rather than juggling an extra one.