Especially if you're studying a different area of study from the previous uni.
So for example if I were to study biology (first-year) and some other subject at Monash, but wanted to transfer to Melbourne to also do a BSc and choose another area of study, would I be putting myself at a huge disadvantage to other students seeing how I've pretty much missed out on a semester's amount of work?
Also, when doing mid-year transfers, do they still look at your ATAR score and subjects completed in VCE? Cause I don't fulfill the prerequisite requirements to do a BSc at Melbourne (hence I plan to study at Monash first but they don't have the area of study I want)?
Since Monash and UoM are Go8 unis, there is an agreement between all Go8 unis that you can get full credit for the subjects studied at either uni. So when you transfer to B.Sci at UoM, you can get credit for biology and your other subject if you were studying part time at Monash.
As for the disadvantage, not really, because since you've done biology at Monash already, I'm sure they'd let you do the second semester biology subject with the other students. If not, then you wouldn't be disadvantaged, you'd just be studying with a different cohort each time.
With mid year transfers, if you've done less than a year of tertiary studies, then your ATAR will have more weighting. So you'd need to fulfil the pre-requisite for B.Sci at UoM before applying.