While HSC economics would help, it is far from being essential.
Mathematics, however, is very important. Particularly if you are planning to major in economics (and even more if you pursue honours and/or postgraduate research).
While some people may manage with just general maths, I would strongly advise against it. Generally the more comfortable with mathematics you are, the more accommodating studying economics at university will be. Too many people develop the perception that economics in uni is like that of economics in the HSC (i.e. lots of rote learning, memorising, wish-washy explanations and no maths at all [though to be fair, there are political economy majors, but honestly I'd view that more as a history/politics program than a proper rigorous economics program]), and end up doing badly or barely passing first year economics (USYD's introductory microeconomics course has a 20% failure rate because people can't understand the maths [which in itself isn't difficult, but confidence in understanding mathematical expressions and equations is something many students lack]).