I'm going to assume you're talking about clinical science. So, there are core subjects that everyone in clinical science will share regardless of your focus. In semester one of first year, these will be: Physiology, Molecular Basis of Life and either General Chemistry or Chemical Structure and Bonding depending on the Chemistry stream you choose. The only difference in first semester is that if you choose medical science you'll end up doing an MDSC topic called Skills for Medical Scientists and if you choose health science you'll end up doing a HLTH topic about the Australian healthcare system. The MDSC topic basically introduces you to skills that will help you conduct research - you learn about experimental designs and you actually get to write one yourself. You also learn how to write a literature review and a research proposal. You'll learn oral presentation skills too, and there is a lot of group work. The HLTH topic on the other hand teaches you about the role of healthcare professionals and how the Australian healthcare system works (I chose medical science so I can't speak too much about this topic).
In semester two, everyone will do Evolution of Biological Diversity, Legal and Ethical Aspects of Health Care and either Modern Chemistry or Chemistry for the Life Sciences. The difference between medical science and health science in semester 2 is that you'll do a communications topic in medical science (purely an English topic where you basically learn how to refine your writing) and a contemporary health issues topic in health science.
So I guess the main difference between health science and medical science is whether you want to learn a bit more about research or whether you'd prefer to learn about the Australian healthcare system and different perspectives on health. There are fewer exams in the health science focus if that appeals to you. One benefit of medical science, however, is that in second year you get an elective in semester 1, whereas people in health science don't (everyone has two elective slots in semester 2 though). The majority of people actually choose medical science but both are equally as relevant in preparing you for the MD given the majority of topics are basically the same across both focuses; moreover, you can tailor your electives in second year to help you prepare for MD. But you can literally choose any elective you want (e.g. a language) provided you've gotten approved.