If your dad has a PhD in chemistry, here is one simple fact he should be able acknowledge:
Year 12 Chemistry education is crap and terribly uninspiring to most.
I loved year 11, then hated year 12, and now I want to get an extended major + honours and hopefully PhD in the area. If you did not like year 11, and are not looking forward to year 12, do not do year 12 chemistry.
Maybe meet your dad in the middle - by taking biology as opposed to health, you can still do a science degree at Monash or Melbourne. If you then decide you want to go into biomedical science, you can then still do this through a science degree + careful unit selection, and even pick up chemistry in first year university if it looks like you'll need it. You DO NOT need year 12 chemistry to do chemistry at an undergraduate level.
EDIT: Should probably be careful with the bold part - if you have a good teacher, you can still enjoy year 12 chemistry. However, quite a lot of my friends and myself hated year 12 chem, and all of us want to get at least a minor in it now. That might say more about the excellence of the chemistry program at Monash than year 12, though.