Assumed knowledge means that what they expect you to have, but its not required to join the course.
If you have the required ATAR into the course, you will make it in regardless of the assumed knowledge.
I had a background of General Maths in school and my current course states that the assumed knowledge was 2-unit Mathematics for B.Science/B.Business. I still made it in regardless of my high school subjects.
Although if you're doing General Maths in school and want to study say Engineering, you will struggle in this because the assumed knowledge is at least Extension 1 Maths. However, this won't prevent you from studying this.
At the moment, I believe the only university that has prerequisite in metro-sydney is Macquarie University for it's Actuarial Studies and other heavy mathematical courses. Other major universities like USYD, UNSW, UTS and UWS don't have this.
At the moment, if you meet the cutoff for USYD's course, regardless assumed knowledge, you'll be admitted into that course.
Just do what's best for you right now.