dux at my school was determined based of highest average mark which is unfair cause the person who got dux did visual arts, cafs, business and drama and the actual smartest kid in our cohort who did 4u math chem phys, came third despite him getting a 99.30 atar in the end. I can guarntee you...
My school gave out a survey via email asking for our atars, which im pretty sure is the only way for the school to determine what atar you got, as for assessment marks im not 100% sure but i think the school knows them all.