In an article titled The Law and Art: Bill Henson - Bill Henson's Naked Photo Exhibition Causes A Stir In Australia, he said:
The question really is not on the artistic merits of the works, but how far can you go justifying actions on the pretext it is art. If Henson is allowed to circumvent the law does this create an ugly legal precedent where people primarily interested in the sexual exploitation of the young can claim immunity “because it is art.”
Henson probably had consent from the models and their parents or legal guardians. However, the laws were set up to protect the innocent and the vulnerable that may not be properly make an informed decision about posing for a well-known photographer.
I have heard that in law permission cannot be given for an illegal act.
One of the roles of art is to question established truths and dogmas. This Henson exhibition is objectionable, not because it shows naked people, but because it could open the door to the sexual exploitation of children. Fine art adult nudes are a well-respected and long running photographic genre.