Australia was "undoubtedly the best team" a decade or so ago. That is not the case now. Remember how they performed when Smith and Warner got banned for a year? They don't have the depth in player pool, when compared to either India or England. Gone are the days when Glenn McGrath, Brett Lee and Shane Warne would gang up to destroy opposing teams. They don't have a good allrounder of the caliber of Shane Watson. And finally, although I agree that Steve Smith is perhaps the best test batsmen nowadays, his captaining skills are not as good as that of Michael Clarke.
If you compare Bangladesh with Australia then Australia is still far better. This was a one-off match and Bangladesh played far better than the Australian team. Wait for the next match and Australia will try to come back. I am sure Bangladesh knows it and they would be prepared for it.