I'm not too sure about the 75% average (if it's semester based, meaning that you need to maintain an average of 75% per semester, or an overall average of 75% for your undergrad) but after doing one unit of Commerce (introductory microeconomics) I found it quite easy to score well if you attend the tutes and do a couple of practise questions / exams at home. I've never done any commerce before an I copped a 95 for that class by following that method.
If it's semester based GPA, and you go below, I reckon you will lose your guarantee but you should make sure to ring up UniMelb and get their opinion on it.
If you've got a guarantee placed by getting a 99.00+ (not 99.90+) go to Monash law I reckon. That way you are guaranteed to graduate with a law degree. The 99.00+ guarantee is only a FFP guarantee meaning that it will cost you 100,000 for that JD degree. Quite pricey in my opinion.
EDIT: I'm sure that Commerce units will get harder from 2nd year onwards. A 75+ isn't hard to come by, but it is definitely not easy to achieve. You have to put in the work to achieve the marks.