Hi, ext 2 would be helpful, but not necessary as you're taught the ext 2 topics in the uni maths courses. Chem isn't really important unless you choose a materials or chem elective. HSC Physics isn't actually that much relevant to uni engineering, in fact, HSC Engineering Studies is most closest ( plus ext 2 maths). You'll need to know the maths part of mechanics and space topics, there's almost no content involved unlike HSC physics which has a lot of content. There's an electrical course you need to do, but I guess you won't need to worry about that now. So basically, the maths in engineering studies is closely related for at least first year mech engineering.