Speaking for Physics/Chemistry, prelim content is assumed knowledge according to the syllabus but this is pretty vague. In my opinion, you can be expected to know the 'overarching' ideas of the Preliminary Course, for Phys an appropriate example would be something like momentum and impulse-if you look up the HSC papers you can see momentum being tested as a part a) of an I2I question (sorry forgot the year) but at the same time momentum is not even mentioned in the HSC Phys syllabus so therefore this is 'assumed knowledge'. For Chem, examples would be calculations with moles would be the example. But as I stated earlier they only expect you to know the 'overarching ideas' and not some obscure dot point that has no relevance eg. history of Galvani and Volta. To be 100% safe with the assumed knowledge, I do not recommend revise prelim content once HSC starts but rather put in the effort during Prelim to gain a holistic understanding so the knowledge becomes part of your long term memory-who knows you might need these broad concepts one day in Uni as well-so all in all take the Prelim subject SERIOUSLY if you want to have a no-risk HSC