Your medium is not crucial as band 6 responses come from a whole range of mediums. However, medium can become an issue in regards to money and technical competence, if you are creating a video or radio play it is going to require more money and technical expertise than a play , story or speech. So your skill set has to match your medium. Also if you look at the notes from the marking center for the 2012 HSC having a conceptual framework is absolutely essential and you also need to ensure that your major work is an extension of your work in Advanced and Extension 1, and it must go beyond the parameters of these courses, you have to show evidence that you have researched independently. You also need to show clear evidence of purpose and audience. These are attributes of a band 6 response.
The Extension 2 course is something i have always been very passionate about and one i have always enjoyed teaching. If you would like to, i would be more than happy to guide you through your extension 2 coursework. Whether that be through online SKYPE and email support or Face to face tutoring sessions (Depending on your location). Or possibly even a combination of tho two. If you would like to get in touch with me my details are in my signature.