For the pre-requisites, you will need to print out the subject guide of the vce equivelant uni subject at your uniand send it to melb uni to review whether they will accept it as a substitute or the vce subject.
As for scores required to transfer; this yrs "competitive" average for transfer was 75 - however this will fluctuate very yr depending on demand. Having said that, the fluctuations are generally very small so a 75 should usually put you into consideration.
For mid yr transfers after doing a semester in 1st yr - melb will weight your atar and your marks achieved at uni (roughly 50/50). Therefore, if you achieved a high atar (ie. just below the cut off) than your uni marks will need to be around 75. A higher atar than that would then mean a lower required uni average. And therefore consequently, a lower atar will require a higher average. As for how this weightage is calculated: nobody really knows.
My advice is to study hard, get a good grade and apply mid yr. if you dont get it, you can always try again after 1st yr (no harm in repeated efforts).