In January you will receive an offer for Melbourne Commerce, at which stage you will accept. You then get the opportunity to apply for a 'second round offer' at which stage you can change preferences to have com/law first. If you get an offer you'll be able to do law.
Unfortunately, that is incorrect.
If you've met all the pre-requisites for Commerce at UoM and looking at the subjects you've done in your signature, I believe you have and you have achieved higher than the Clearly-In next year which gets released at the same time as offers do, then you will be offered a place for your first preference. As a result, you will not be considered for any offers in subsequent rounds.
If you that does happen, and you really want to do Commerce/Law at Monash, then you must check that you have:
1. Met all the pre-requisites.
2. Achieved over the Clearly-In
If you have done that and you've passed both hurdles, then you apply for an Irregular Offer. For more info on Irregular Offers, check
this out.