If a denial of procedural fairness has deprived the applicant of a fair hearing, there is no need for the applicant to prove anything more. This is contrasted with the situation where a denial of procedural fairness did not otherwise result in a fair hearing (probably a rare situation).
See discussion in Minister for Immigration and Border Protection v WZARH  HCA 40; 256 CLR 326: per Gaegler and Gordon JJ at -:
The concern of procedural fairness, which here operates as a condition of the exercise of a statutory power, is with procedures rather than with outcomes. It follows that a failure on the part of an assessor or reviewer to give the opportunity to be heard which a reasonable assessor or reviewer ought fairly to give in the totality of the circumstances constitutes, without more, a denial of procedural fairness in breach of the implied condition which governs the exercise of the Minister’s statutory powers of consideration.
Such a breach of the implied condition which governs the exercise of the Minister’s statutory powers of consideration is material, so as to justify the grant of declaratory relief by a court of competent jurisdiction, if it operates to deprive the offshore entry person of “the possibility of a successful outcome”.
That approach to the determination of the existence and consequence of a breach of an implied condition of procedural fairness governing the exercise of a statutory power is wholly consistent with the often-repeated observation of Gleeson CJ in Re Minister for Immigration and Multicultural and Indigenous Affairs; Ex parte Lam that the concern of procedural fairness is to “avoid practical injustice”, and with his Honour’s conclusion in that case that there was no denial of procedural fairness where “[n]o practical injustice ha[d] been shown”. The absence of practical injustice in Lam lay in the fact that “[t]he applicant lost no opportunity to advance his case”; it was not “shown that he lost an opportunity to put any information or argument to the decision-maker, or otherwise suffered any detriment”.
Contrary to the submission of the Minister in this appeal, and as has repeatedly been recognised in the Full Court of the Federal Court, Lam is not authority for the proposition that it is incumbent on a person who seeks to establish denial of procedural fairness always to demonstrate what would have occurred if procedural fairness had been observed. What must be shown by a person seeking to establish a denial of procedural fairness will depend upon the precise defect alleged to have occurred in the decision-making process.
There are cases in which conduct on the part of an administrator in the course of a hearing can be demonstrated to have misled a person into refraining from taking up an opportunity to be heard that was available to that person in accordance with an applicable procedure which was otherwise fair. To demonstrate that the person would have taken some step if that conduct had not occurred is, in such a case, part of establishing that the person has in fact been denied a reasonable opportunity to be heard.
Where, however, the procedure adopted by an administrator can be shown itself to have failed to afford a fair opportunity to be heard, a denial of procedural fairness is established by nothing more than that failure, and the granting of curial relief is justified unless it can be shown that the failure did not deprive the person of the possibility of a successful outcome. The practical injustice in such a case lies in the denial of an opportunity which in fairness ought to have been given.