I'm no expert on this, but my sister just started Food science at UNSW this year, so I've researched a little in an attempt to calm her down a bit about various things over the summer.
For subjects, definitely Chemistry, probably Biology, and I imagine Food Tech or whatever the HSC food course is called. Personally, I've found having 2U+ maths knowledge to be really handy at uni, even if it's not directly related, and PD/H/PE is probably not a bad choice considering you're interested in a health-related field.
Food falls under engineering at UNSW, which means you're eligible to apply for FEAS. If you anticipate being below the cut-off, you apply and can get in through an interview.