I think we're in heated agreement, but using different words, AP! You are of course correct. The High Court's 'main' job is to decide whether 'something' is lawful or not (or, in layperson's language, "legal" or "illegal") contrary to Eclan's ridiculous assertion.
In this case, the High Court will decide whether the Full Court of the Federal Court's judgment contains errors of a kind that would justify allowing Qantas's appeal. That process necessarily entails the High Court first deciding what the applicable law is and means.