What is the legal age for nude photography in Australia?

It all depends on the nature of the image, if it's a shot of a naked child running the sprinkler, or a bath-time photo, it is typically legal to do so. as long as the photos are not of children engaged in sex acts, or posed in a sexual manner, you should be fine, even if genitalia is visible.

If you want to see an image that would be illegal in most states, look up "virgin killer" by "the scorpions" for the original cover, and compare it to the cover of "never mind" by "Nirvana", and you should get a real good sense of what type of photo would be legal and illegal.

even something you think that looks innocent, like two naked toddlers hugging each other, regardless of setting, could be deemed as a sex act in some or most jurisdictions.

I am by no way an attorney, and the advice above is based upon my own research.