take a look at the new course structure of melbourne uni though - you might or might not like it - but i think commerce is still there as an undergraduate degree because there are a few professional mainstream degrees that are not longer there but now offered as 2-yr masters after u finish your 3 yr degree - but melbourne uni for sure since im from there lol

Yes , it is harder to get in simply because there are more prospective students who wants to get in there and uni's know that international students have unlimited amount of money so they are really setting their enter requirements higher .

It's too early to say you will be surrounded by smart/good learning environment - but I can guarantee you the lecturers in Melbourne U are of the highest caliber - the reason stopping them from being even better is if they set say the exams harder - most students would fail (its a business after all)

I'm in my final for a double degree and happen to be surrounded by peers with really high aspiration and work rate and all of which have gotten themselves a shiny job offer 6 months before graduation (4 for me since i sucked a bit more) - BUT there are really a lot of really bad students around here too because either they simply do not have the kind of hard work that a uni life would demand and there are also a lot of them who can barely speak English and always end up speaking in their native tongue (saying in general because it is not limited to just one country) - This does not apply to students who do Commerce though - the work load for commerce is ridiculously low :|

World Ranking shouldn't be a consideration for you if you want to look into getting either one but a good idea if your results are great and finance is not a problem - but it is good to look at break downs of the ranking scores where one of them would definitely be teaching environments - there is also a ranking out there that only rank universities for their faculties - now I can safely say Melbourne has bypassed Monash in every department even in Engineering cause Monash was actually better in Engineering a couple of years back.

but in the end, it is really how the student carry him/herself in a uni environment - if you choose to relax and visit places and enjoy life , then stay in Malaysia - I'm not going to be one of those who will tell you to have fun during your uni life - you can enjoy it but often you will find yourself needing to enjoy hardships and stresses - and its these things that prepare you for the gruesome employment market we are having today

I'm doing Civil Engineering and Commerce so I'd welcome questions if you have em

Would like to correct your statements, no offence. 
QS Word Universities Rankings (JPA recognizes)
20 : Australian National University (1st in Australia)
37 : University of Sydney
38 : University of Melbourne
61 : Monash University (Not the last of G8)

I don't think it's easy to get into Monash Commerce, you're required to get min ATAR of 90.
If I am not mistaken, my friend told me you're required to get min ATAR of 92 for Melbourne Commerce. Not much difference. 
Course is easier at Monash, I don't think so.

Some of Monash graduates tutor Melbourne students, my sister-in law's brother was the senior lecturer in the dep of Medicine of UM, just FYI, related or not IDK.

Good Luck! I support your choice.