Generally speaking, it is normal to forget course content (this applies to all subjects). This is typically due to the fact that a gradual reduction in interaction with particular content/concepts (which is usually accompanied by a focus on more current content at a later point in the year), whether it be less revision, preparation or study that targets the content in question, will, in most cases, lead to temporary alienation (or "hibernating information" as you mentioned) from the content. A simple analogy would be that of someone learning a language. If at some point after having made considerable progress, the person decides to learn another language, while disregarding the language that they have originally studied (i.e. ceasing all forms of interaction with that language), they will likely forget elements of it as time passes.
Based on this, it is important to ensure that you allocate a bit of time towards regular revision of concepts/topics that have been previously covered, which will ensure that you maintain your familiarity with the content. Consequently, you will be in a better position in terms of your knowledge of the content, which can in turn reflect your readiness for your HSC exams. Of course, the method used to do this will depend on the subject(s) in which you are facing this issue.
I hope this helps!
