While that may be advantageous prior to taking up the course, but its not something that is necessary. It's a course where getting an E4 (at least in the examination) would certainly require for you to read outside the textbook and the blue book of readings (its basically a compilation made up by the board with extracts from various historians and social commentators about history). This may sound really weird, but the difficulty of it depends on how your brain is wired. If you are inclined towards the humanities than I think you will find the concepts easier to grasp even if you don't have any prior knowledge. However, its different for everyone.
I suppose that you have seen on here are the history projects. These are 2500 word essays that are handed in, so you do not have to know the information contained in it for any exams, although there is no rule to say you can't write about sources from your project in the test. In the end, if you love history its a subject definitely worth picking up. Personally, it really opened my eyes as to the contextual and philosophical issues involved in the construction of history. Before, I just assumed it to be a panoply of facts we know for certain and disputed facts. History extension makes you realize history is so much more than that and allows you to better appreciate it as an evolving and complex discipline.
I think there is no harm in having a go. It's not for everyone, since its not the traditional 'history' subject that people would have encountered in primary and high school. With the HSC being so flexible, I think you are better off trying, finding out you don't like it and dropping it than never picking it never knowing if you would have liked it or been good enough. But be wary, there whole course is pretty much essay writing.
Best of Luck
