While the AAT is permitted to act in an inquisitorial manner, it must do in accordance with law

The Administrative Appeals Tribunal is permitted to act in an inquisitorial manner. However, if you have a case where the Tribunal has exercised its jurisdiction widely, it's worth checking that the Tribunal made its intention clear and gave the applicant a specific opportunity to address the topic. In S395/2002 v Minister for Immigration and Multicultural Affairs [2003] HCA 71; 216 CLR 473; 203 ALR 112; 78 ALJR 180 at [39], McHugh J stated:

“On a number of occasions this Court has said that proceedings before the Tribunal are inquisitorial in nature. The arguments and evidence of applicants or the Minister cannot narrow the Tribunal's jurisdiction to investigate the generality of a claim for a protection visa. Whatever the arguments or evidence of an applicant, the Tribunal is entitled, but not bound, to look at the issue generally. If the Tribunal elects to exercise its jurisdiction more widely than the applicant or the Minister has asked, however, it must do so in accordance with law. . .”