I'm not sure what the law is in Australia (though according to Yahtzee it's all wonky) but the page doesn't tell us all of it I think. Did the man have regular child porn as well and they only used the cartoon stuff to add to his sentence? Was he on parole? If he was on parole, then parolees (here in the US) are sometimes forbidden from having any pornographic material or it will violate their parole (depending on what they were charged with), which would include cartoon pornography.
If it was just some drawing he got arrested for, then yes, it's pretty dumb. Child pornography laws are made to protect children, not to punish people for their urges. If no child is being harmed, it's not illegal (though some people will obviously disagree and call it a loophole)