It depends what you want to achieve. I assume when you say that you want to do Com/Law, that you want to have an professional entry law degree under your belt in order work in a legally oriented role, or even practice. In this case, you will need to be looking at doing a JD (either at Monash or UMelb), or a graduate entry LLB which Monash offers as well.
An alternative is to do commerce at Monash, and then internally transfer into Com/Law after your first year. It is common for people to do this, but requires good marks (high 70s, or 80+ if you want to be safe).
I know that Monash does actually offer the 'Master of Commercial Law', which is for non-law graduates. But this is a different course to Master of Laws (LLM) specializing in Com, which as Strawberry correctly said requires a professional entry law degree (LLB or JD).
However, if your aim is pursue a career based around your Commerce degree, this 'Master of Commercial Law' could be a useful ace up your sleeve to differentiate you from other Commerce graduates. A 'Master of Commercial Law' will not get you a job as a lawyer, or a legal position in any capacity.