Internal transfers are easier in terms of transferring credits. External transfers are more difficult because obviously sometimes the subjects don't match up properly.
Internal transfers into Monash law require at least a distinction (70) average but I would advise you to aim for 75+ because it is highly competitive.
I don't know what the external transfer requirements are. If I had to guess I would guess that they are slightly higher than internal transfers because Deakin is not in the "Group of 8" and therefore Monash might see their law course as being of a lower standard.
I know Monash has this view because I have a friend who transferred from VU law and had to repeat first year because Monash would not recognise any VU units (lol)
1) If you are determined to do law at Monash, particularly if you're hoping to be a hotshot barrister and therefore need a Group of 8 alma mater on your resume, then put down Monash commerce and work your arse off for that GPA.
- if you don't get the GPA requied you may have to consider LSAT -> JD after you finish commerce
2) If you just want a law degree, put down Deakin (their law school is not that bad anyway) and work your butt off for a high GPA and good extra-curriculars.
I think 2) is a better choice because you can still try to transfer to Monash after your first year, and even if you don't succeed you'll still have a law degree at the end.
If you choose option 1) and don't get into law, you may have to resort to doing the LSAT (which is tougher than transferring IMO) and going for the JD.