Well since your set on MED in general as Shadow said why not do B Medical Science. If your a high performer during the first 1-2 years of the course it can be a direct pathway into undergrad medicine at UNSW.
But if you are wanting something to "fall back on" it is questionable to whether commerce/business would be a good option since most of the "commerce" type jobs don't technically have a "commerce" qualification requirement.
Sci/Bus (science faculty) = 3 years with significantly less options for majors whilst Com/Sci (business faculty) = 4 years with greater variety. So it comes down to what you want to do. That said you could do Sci/Bus then apply for postgrad med. If you fail that round do honours (+1 year) and apply again.
Also you could apply to undergraduate medicine even after you receive your degree if you really are that passionate.