Doing HSC is extremely beneficial, regardless of whether they're similar or not. The style of questions will set you up for studying for the HSC. Depending on your school, and what exam they set, the questions may or may not be similar to those in your internal exams/trials. For example, Sydney Boys and NEAP tends to follow the HSC style questions, whereas Catholic and Independent papers can be a little weird and different, with a few questions being dissimilar to those expected in the HSC.
As for repeating questions, yes, the syllabus for particularly Chem and PHys is quite limited, and hence, it only makes sense that every few years, they will cycle the questions around. A question that was perhaps 7 marks will only be worth 3 marks in a later year (different verb: 'assess' for the 7 marker, whereas it'll be 'explain' for the 3 marker).