I may be a bit biased since I study and CS and maths, but you shouldn't let your current math skills deter you from pursuing computer science. Also, it may be possible that you will enjoy maths later on and the current way it is taught in high school just doesn't work for you.

Although it is useful to have done 4U for first year maths, it's not necessary (but without 3U most would struggle).

For UNSW computer science, there are three compulsory maths courses in first year - see

http://www.handbook.unsw.edu.au/undergraduate/programs/2013/3978.html. They are challenging and build on your previous skills, but it is definitely doable as long as you work hard at 3U. I know people who have gotten through these courses even without 3U (though it takes more work).

The rest of computer science, there will be less of the calculus-based topics -

**the more relevant 3U topics are: algebra, probability, polynomials, binomial theorem, permutations, combinations, series and applications.** They won't always be used directly, but the concepts are important to understand. So depending on which topics you are best at, this may work better for you.

The reddit threads that Flop21 posted are pretty good.

In reply to the commerce thing, from anecdotal experiences commerce is much more saturated with graduates vs. the level of demand, compared to comp sci graduates. Some quick stats from Good Universities Guide seems to confirm this:

http://www.gooduniversitiesguide.com.au/course/view/Bachelor-of-Science-Computer-Science/profile/UNSW-Australia/(StudyField)/Computing and information technology/(Qualifications)/M/(Course)/computer science
http://www.gooduniversitiesguide.com.au/course/view/Bachelor-of-Commerce/profile/UNSW-Australia-Business-School/(Qualifications)/M/(Course)/commerce
Anyway you can always transfer if it doesn't work out, I highly recommend giving comp sci a shot

(But work hard at your maths.)