Under s 67A of the Crimes Act, child pornography means:
a film, photograph, publication or computer game that describes or depicts a person who is, or appears to be, a minor engaging in sexual activity or depicted in an indecent sexual manner or context
The section provides that a minor is:
a person under the age of 18 years.
Last year in R v Clarke (2008) 100 SASR 363;  SASC 100, the Court of Criminal Appeal affirmed Kennedy, and decided the age of the person depicted in child pornography is a strict-liability element.
In Clarke, the accused was one of three people who procured two 14-year-old girls to produce pornography. Clarke honestly and reasonably believed two girls were over 16, the prescribed age for the South Australian provisions. (The trial judge, and the Court of Appeal, both noted the girls not only said they were over 16 but also looked it, though they were in fact under 16.)
Clarke’s honest and reasonable belief was no defence to the charge.
[T]here will be cases when the age or appearance of the person involved in the pornographic material is such that there is room for uncertainty as to whether a child is involved. But one can understand that Parliament might enact legislation the effect of which is to punish those who do not realise that they are involving themselves in child pornography, even though they are. The reason for taking that approach would be that the suppression of child pornography is sufficiently important to punish not only risk takers (those who do not have an honest and reasonable belief that a child is not involved), but also those who do have an honest and reasonable belief that a child is not involved. The justification for punishing the latter group would be, on this hypothesis, that the suppression of the production of and trade in child pornography warrants such a stringent approach: R v Clarke at  per Doyle CJ