First semester of uni chemistry (CHM1011) involves a lot of new material not found in the VCE course, as one would expect. It's a lot to do with physical chemistry as you go through atomic structure (and the various theories), bonding, thermodynamics, equilibria (which we have just finished) and kinetics. Second semester is organic chem, which by the looks of it, overlaps with VCE more.
The workload is really minimal although a lot of content is covered in a short space of time. There aren't any assignments - only pre-lecture tests and post-lecture tests (the latter of which you get 3 attempts, and the highest score is taken) and none of which are timed. They're done online and open book; each test should only take ~10 mins.
The exam is in a month-or-so and although I'm nowhere near ready, past students have told me that they literally crammed everything a few days before the exam and still managed to pull HDs. Hopefully this is true haha.
I think as long as you're not so serious about the subject, it shouldn't require too much effort. It's a bottom two subject after all. But I suppose if there's no reason other than for interest's sake, then maybe doing a uni subject IS going to add to the year's workload considering you'll already be undertaking five.
Personally, I chose to do an extension subject because a) I wasn't keen on doing VCE Physics 3/4 b) I wanted to do a subject more interesting than a VCE subject could offer c) free periods are nice
The best thing you could do is apply, get accepted and just try the program for a couple months and then decide whether you'd like to continue or not. Payments are due a couple months in so if you decide it's not right for you, then you can easily withdraw without incurring any fees.
Hope this helps!