On the face of it seems to me clear that the fact led by the Crown that they had pleaded guilty goes to and only to, in this case, the question of the credit of these two young women. In other words, the Crown wished in-chief to bolster their credit by suggesting that having been dealt with they had no motive for trying to shift the blame from themselves to somebody else. This, of course, in certain circumstances is legitimate although such evidence cannot be conclusive because, for instance, the plea of guilty may have been a result of plea bargaining below...
The circumstances when this sort of evidence can be admitted are these: it is often put to a witness, who has been an accomplice or apparently been an accomplice, by the defence in cross-examination that they have been charged with and not dealt with for this same offence and, in those circumstances, of course, the defence suggest - and it is a perfectly proper and very strong suggestion in some cases - that the witness has a very strong motive for shifting the blame from himself or herself on to the accused.
In certain circumstances when such cross-examination is conducted, it is clearly open to the Crown to prove, if they can, by re-examination that they have been dealt with already by way of rehabilitation of the witness' credit. But in this case no such thing occurred. Accordingly, it seems to me that this evidence was led as to credit and nothing but credit, and on very well-known principles evidence which goes only to credit cannot be admitted in proceedings of any nature.
There is authority in New Zealand which suggests that the proposition I have just suggested is correct. In The Queen v Windsor, the Court of Appeal in New Zealand held...
Whether, therefore, the plea of 'guilty' be regarded as an act or declaration, evidence of it was not admissible against the applicant on his trial... If the evidence of a plea of 'guilty' was neither a declaration admissible against the prisoner nor an admission or confession of which evidence could be given against him, then it is difficult to imagine on what ground it could be received. Its real purpose was, we should imagine, to reinforce the evidence of [the co-accused] by informing the jury that she herself had acted to her detriment upon the truth of the matters alleged in her testimony. But evidence of that could scarcely be admissible against the appellant, for what [the co-accused] said or did when a common purpose had ceased to exist was not admissible against the appellant.
...
Accordingly, I am of the opinion that the fact that the two witnesses pleaded guilty was irrelevant and inadmissible and should have been excluded.[12]