In Canada, it is mandatory to report child abuse if you suspect that it is occurring.
The complaint, in my mind, is not with the lab, but rather with the final determination.
On its own, a photo of a naked child isn't conclusive or even good evidence of abuse. In Canada, I would say that the authorities should have made their determination that no abuse was indicated, and then returned the transparency to you.
The rules in Australia may be unreasonably different.