Is the first year truly a 'common' first year? According to a few course guides, the subjects listed are the same regardless of major (I believe the exception is chemical/biomedical). So if this were the case, would I only have to choose a discipline (civil, mechanical, electrical etc.) after the first year?
Secondly, how difficult is the course itself relative to other uni's? I'm tossing up between this and Monash.
Pretty much. Like you say chem/biom is the only exception with ESD1 as a prereq. The rest of the streams will all need ESD2 instead. If you're doing Commerce you will need to decide when you enter second year, as you won't be able to take all of the subjects if you decide to switch. Science students can leave it up until third year if they wish.
Difficulty of course is incredibly hard to measure, given it is going to vary between subjects, student, lecturers, years etc etc.
I've not heard of Monash or Unimelb being significantly easier/harder than the other if that means anything.
One of the points to consider is that BEng at Monash is 4 years whereas the equivalent at UoM is BSci/MEng for 5 years.
There is also the 65% minimum WAM needed for masters entry but realistically this isn't too difficult if you go to class and do some modicum of study before exams, in first year at least.