I would be careful about this. As others have suggested, you need the advice of an immigration attorney that is well versed in US immigration law, and perhaps a separate one regarding Australian law.

My concern about taking the above quoted advice is that now that you have a US immigrant visa, it may not be possible to legally enter under any other status without first canceling the immigrant visa (or having it automatically canceled when you enter on another status). Because of the special situation with your possible change of nationality, it may not be caught, at least not right away, as the US may have no way of knowing that the Indian and Australian passports are for the same person (will your Australian passport have the exact same name as your Indian one?). Also, there's the matter of fingerprinting upon entry that may end up revealing that the Indian national and Australian national are the same person. And even if it's not caught right away, if you plan to apply for permanent residency status once you arrive, and/or eventually naturalize, this issue is likely to come up. And once you present yourself at a US Port Of Entry and apply for admission with a certain status and nationality, you can't undo this historical fact later and it might come back to bite you. Be careful.