I transferred from RMIT University B. of Business to Melbourne University B. of Commerce this year. So I can give some advice on the required scores for a successful transfer (even though it is in a different field, the transfer requirements are similar across all faculties).
If you are to transfer midyear, UniMelb will take into the marks you achieved in latest 2 semesters of study. Therefore, your ATAR and your first 4 subjects studied at Swinburne. I would advise for you to study a full year at Swinburne before applying for a transfer as an atar of anything below 90 would harm your chances of gaining entry (even if you achieve 4 HDs at Swinburne). Having said that, there is no harm in applying through VTAC at the end of 2012 - UniMelb won't look negatively on your application later on even if you fail the first time.
As for the 70 requirement, be careful about this threshold. The 70 is a MINIMUM score to be considered - in fact, I believe this threshold has risen to 75 (according to the student adviser I talked to and on their website - search for entry requirements for future students on the Unimelb website). 75 would be competitive for transfer from Swinburne science to Melb Science - however, it may not be enough for a transfer from science to BioMed. This is because, the requirements are based on the number of students competing for spots in the particular course/program; and BioMed is a VERY competitive course (99 ATAR in 2012) which means you are competing against very smart cookies from all around Australia hoping to gain entrance into the program (plus UniMelb will give preference to kids who apply from from other courses within the Uni).
As a yardstick, I gained entry into UniMelb commerce from RMIT with a 86 average after completing 2 semesters of Uni.
May I inquire as to why you want to transfer to Melbourne? Undergrad degrees are quite similar throughout Australia and job prospects would be similar upon graduation (provided you achieve a high enough mark). The only reason I transferred to Melbourne was for their superior postgrad programs (and I wanted to maximise my chances of getting into the Economics Honours program).
EDIT: Also, you have to satisfy the pre-requisites needed for the course. (ie. study score of 25 in English and Methods for B.Commerce - not sure about Science or Bio-Med). You can find them on VTAC or on the UniMelb website -http://www.futurestudents.unimelb.edu.au/admissions/applications/other-applications/mid-year-entry/mid-year_entry_2012_undergraduate_domestic_students