Birds of Victoria - by Damian Kelly
The white under-tail confused me, because the field guides tend to show the hardhead / white-eyed duck as having the white under the tail, and not the blue-billed. But further research gave one book showing some white under the tail of the blue-billed.
With the stiff-tail clear in this image (my checking indicated only the musk duck and blue-billed are stiff-tailed ducks in Australia), I concluded that you are right and I am wrong.
So field guides should mention rear views - we see them often enough!