Well if you do Maths 1A (MATH1131) as required by most (if not all) eng courses, it's roughly equivalent to 4U HSC maths (maybe 3U at it's very easiest). Of course it has quite a bit of extra stuff (e.g. vectors, matrices, more intensive calculus), but these aren't too hard to pick up provided you study & revise regularly (not cram too much).
If you choose to do Higher Maths 1A (MATH1141), you learn a little more topics than MATH1131 students learn, but in the main difference in Higher Maths is that the questions in the exams are harder & more challenging (the exam consists of 4 questions in common with MATH1131, and 2 extension questions). To do Higher maths, you must have done 4U maths, and have a HSC mark of about 80+. Because Higher Maths is quite alot harder, it gets scaled so that Higher maths students aren't disadvantaged (sort of like 4U maths at high school).
Discrete maths (which you will do if you choose elec/telecom/software/comp eng or comp sci) is commonly seen as the hardest first year subject by most students, with good reason too. While some of it may be similar to high school maths/sdd (e.g. induction, probability, logic, algorithms etc), alot of it is new stuff which can be quite obscure & abstract at times, and difficult to understand. I personally think it's harder than MATH1141. Like any subject, constant study & revision is the only way to get a good mark.
And of course there are other levels of maths like General Maths, and Maths for Life Sciences, which are easier than MATH1131/1141, which is all some courses like biological sciences require.
Have a look on the UNSW maths website, it's got plenty of good info:
www.maths.unsw.edu.au
What are you interested in doing at uni pri?
[Edit] Also forgot to mention that in MATH1131 & MATH1141 you learn how to use Maple (a computer program) to solve mathematical problems for you. You also have to do an assesible Maple test to see how proficient you are in using it.