The [Crown] obligation to disclose includes, in an appropriate case, an obligation to make enquiries: [R v Garofalo [1992] 2 VR 625,637 (Ormiston JA)]. To lead to a conclusion that a trial is vitiated by non-disclosure it is not necessary for the appellant to demonstrate that the jury verdict would have been different had the obligation been complied with. As Glidewell LJ said in R v Ward [1993] 2 All ER 577, 599:
Non-disclosure is a potent source of injustice and even with the benefit of hindsight, it will often be difficult to say whether or not an undisclosed item of evidence might have shifted the balance or opened up a new line of defence.
...
The High Court in Grey v The Queen [2001] HCA 65; (2001) 184 ALR 593 ("Grey"), considered a situation where an accused had not been provided with material in the possession of police concerning a Crown witness ... [whose] evidence was significant to the prosecution case against him. The material withheld was a 'letter of comfort' provided by a [police officer] ...
[T]he plurality in the High Court (Gleeson CJ, Gummow and Callinan JJ) said that as ... [the witness] had been put forward by the Crown as a reliable witness the Crown case was presented on a disingenuous basis. The fact that the defence did not know of the special relationship between [the witness] and the police when that relationship ought to have been disclosed meant that there had been a miscarriage of justice in respect of Grey's trial. Their Honours refused to apply the proviso to save Grey's conviction ...
Subsequently, the High Court in Mallard v The Queen [2005] HCA 68; (2005) 224 CLR 125 ("Mallard"), again considered evidence which the Crown had failed to disclose to an accused. In doing so the plurality (Gummow, Hayne, Callinan and Heydon JJ) expressed the ratio in Grey's case as being (ibid 133):
... that the prosecution must at common law also disclose all relevant evidence to an accused and that a failure to do so may, in some circumstances, require the quashing of a verdict of guilty.
Kirby J expressed the principle in similar terms, after reviewing Australian and international authorities (ibid):
The applicable principles: The foregoing review of the approach of courts, in national and international jurisdiction, indicates the growth of the insistence of the law, particularly in countries observing the accusatorial form of criminal trial, of the requirement that the prosecution may not suppress evidence in its possession, or available to it, material to the contested issues in the trial. It must ordinarily provide such evidence to the defence. Especially is this so where the material evidence may cast a significant light on the credibility or reliability of material prosecution witnesses or the acceptability and truthfulness of exculpatory evidence by or for the accused.
In Mallard the Crown's failure to disclose concerned evidence in its possession which might have been exculpatory of the accused.