B.Com

I suggest you formerly apply to various colleges in Australia and then apply for visa on the basis of possibility of admission in a college thre. For this the better route would be through a registered agent.

you can purusue b.com with cs course not mba

You can get admission.
Candidate must have passed 10+2 or its equivalent exam from any recognized Board secured minimum 60% marks in aggregate and 60% marks in related subject for general category and 67.50% in aggregate and 54% in related subject for OBC-A and OBC-B category and 52.25% in aggregate and 45% in related subject for SC/ ST category of West Bengal.
https://www.shiksha.com/getListingDetail/119974/course/course-B-Com-Hons-Kolkata-India

there is not much time left , you can still apply in many private universities for post grad. like amity,ansal,dbs. or you can also give mat .

To be eligible for MBA you must hold a bachelor's degree with 50% aggregate.You could go for BBA and start preparing for CAT right from the 1st day of ur graduation so that by the end of 3rd you could face the exam with confidence, having some outstanding scores(say 95+ percentile).You get landed in one among the top business schools.(Remember if your percentage in graduation is above 75% you get shortlisted easily for iim's based on application rating).
All the best :-)