Exactly, which is what I posted earlier - only an Australian Court can terminate parental rights.
But we have no reason to believe that Zahra's mother didn't have her parental rights terminated by an Australian court many years ago. If she did, then she is not a person with "parental responsibility" for Zahra, and AB doesn't need her consent to get Zahra a passport, any more than he needs yours or mine.
We don't know, of course, what her legal status is with regards to Zahra, but given everything we do know, it's not unlikely that she has no legal parental rights/responsibility. While it might not be uncommon for an unstable parent to informally surrender control of a child to the other parent, and not bother with court proceedings, given Zahra's extensive treatment for a life-threatening disease, I think it's very likely that medical professionals and/or medical social workers saw to it that Zahra's parental situation was legally formalized, if it hadn't previously been.
I just think it's very premature to suggest that AB may have taken Zahra out of the country illegally. I expect if he had, Australian consular officials would be all over this case and making formal public statements to that effect. The fact that consular officials weren't all over the matter of Zahra's being here two years ago, suggests to me that the mother is legally out of the picture, and was in no position to demand that authorities track down her daughter. If she had some legal rights, but just didn't give a d***, then she probably would have readily signed any form that was stuck in front of her by AB or the passport-issuing authority and AB would thus have had her consent.