Economics at VCE level only provides an introduction to the theory behind the discipline (i.e. How the Demand/Supply curves move; How Unemployment affects Interest Rates/Inflation/Foreign Exchange rates etc.). There is precious little (if any) mathematics contained within the syllabus.
University Economics will cover all of this, plus introduce you to not only why and how these movements occur; but also some basic mathematical/economic mechanisms that allow them to happen. Concepts like Marginal Revenue/Cost; PPF Curves, MPL/MPK, Elasticity of Price/Demand/Supply, Price Theory in a Competitive Market vs a Monopoly (and even perhaps Oligopolies), Game Theory etc.; which are not covered in High School (at least in my time) will be introduced in 1st year.
However, don't be too freaked out about all these fancy words that I managed to sprout out in this post. They are just fancy Economist speak for very basic mathematical concepts. So if you are competent in Mathematics in high school - this stuff will be ridiculously easy to grasp. It is the students who may not have done as well in Maths that seem to struggle some of the topics.