Dear jehwey,

You have a strong, competitive profile overall, so I think you should start working on reflecting that in your essays. If you submit a strong application, then I think you have a good shot at any school in the 10-25 range. To answer your questions:

1. My grades were quite average in my freshman year but they improved consistently, and I achieved a 80% average in my 3rd and 4th year, does my GMAT score make up for this weakness?
I don't think this is much of a weakness for you. It is a common trend on the transcript and one that schools like to see. Your GMAT score also indicates your academic ability, so in my assessment, you are a strong applicant in terms of GMAT/GPA.

2. the Australian/British grading system is very different from the US, we use, 1st class honours, 2nd class honours...etc. Do yo have some suggestions regarding the convertion?
Most schools are familiar with the different grading systems. However, you will be expected to report an equivalent US GPA. The most accurate way to do that is to get your transcript evaluated by a credentialing service like World Evaluation Services or Global Credential Evaluators. However, that is not necessary if you can do a fairly accurate conversion on your own. Unfortunately, there are no resources I am aware of that are in the public space that have proven to be accurate, but you might search on this forum or some others to see if you can find some possibilities. Likewise, many schools have developed their own conversion scales and posted those in their online applications.

3. The schools im looking to apply to, USC, UNC, Georgetown, UCLA, Duke. I have read that the application is very competitive this year and is my profile good enough for these top 10~25 ranked schools?
Yes, I think you are competitive for these schools. To give yourself the best chance, you want to focus on writing a strong set of essays, securing great recommendations, and submitting your application as early as possible.

Best of luck,
Lisa