Theres a massive difference between prelim and actual HSC. The way I see it, is prelim focuses on the structure and influences on the economy on individuals, firms and governments and how all aspects of the economy work together.
However, with the HSC I've noticed they don't generally always cover those aspects. It more looks into how Australia as a whole (instead of specific functions such as share markets in prelim) relate to the Global Economy and what are the internal issues facing the Australian economy and the policies towards them. HSC course is harder because it derives off prelim functions (such as knowledge in product, factor markets, opportunity costs, models, share markets, government responses, labour market policies, etc) but also it continues into the aspects of specific issues and how they relate to a contemporary society. HSC is more interesting though