I believe you can get into medicine from a bachelor of science (someone correct me if I'm wrong) but it is very challenging.
It's not very challenging - not any more than if you were to do biomedicine, you have the same chance as someone doing Biomed
Also, building on what Paul said, I believe University of Melbourne is the only uni in australia that requires prerequisite subjects for their graduate medicine course. This means that you would have to do Biomedicine, Science or an equivalent undergraduate course while choosing the necessary subjects IF you wanted to medicine at Melbourne. If you're happy with going to any other uni in the state, then it doesn't matter which undergraduate course you do, and which subjects you choose. So you could do Arts at La Trobe and still get into the medicine course at Deakin - or any other university apart from Melbourne uni.
But keep in mind, for graduate medicine, you need to do a test called the GAMSAT. It requires you to know Physics at a VCE level, and Biology and Chemistry at a first year uni level, among other things. So if you're doing something like Arts, it can make it quite difficult for you to do well on the GAMSAT because you'll have to self learn all the content. But there are people out there who do this, but yeah it's much easier if you do a Science related course and pick the appropriate subjects that will prepare you for the GAMSAT if you can fit them in