40 His Honour added that the expression "proper understanding" was by no means a narrow one, and includes a "complete or thorough" understanding, concluding:
However, I think it fair to say that, if a privileged document is voluntarily disclosed for forensic purposes, and a thorough apprehension or appreciation of the character, significance or implications of that document requires disclosure of source documents, otherwise protected by client legal privilege, ordinarily the test laid down by s 126 of the Evidence Act will be satisfied.
41 His Honour then rejected the proposition that inspection of a source document must necessarily be reasonably necessary for a proper understanding of a report, noting that if it had been referred to for a very limited purpose, and that no further reference could assist in a proper understanding of the report, it would not satisfy the test.
42 In Tirango Nominees, the issue arose during the cross-examination of an expert accountant who had produced reports on the economic loss suffered by the applicants, based on letters of instructions from the applicants' solicitor. The witness did not identify in his report anything in the letters of instructions as influencing the opinions he offered. Cross-examining counsel called for the letters of instructions. The claim for privilege was upheld, Mansfield J holding that the calling of an expert witness did not of itself amount to the disclosure of the substance of the instructions given to that witness, although it sometimes may. His Honour rejected the proposition that even at common law, as a general rule, presentation of an expert to give evidence waived privilege in the letter of instructions (at 366):
Counsel for the respondents submitted as a general proposition that, at common law, the document containing the instructions to the expert upon which the expert's report is given is available once the expert is presented to give evidence, because privilege in the letter of instructions necessarily is thereby waived. I do not accept that contention. In the limited time available, no authority directly on point has been referred to.
43 At 367, his Honour said:
However, I do not consider that the High Court determined that invariably once such a document does become evidence all the documents giving rise to its creation are or should be available for inspection. Whether they should or should not be so produced will depend upon whether they do provide any foundation for the opinions expressed by the expert or whether the picture may be misleading or partial only if they are not produced. I agree with the reasons of Foster J in Dingwall v the Commonwealth (1992) 39 FCR 521 on that topic. …
44 Then his Honour referred to what Sackville J had said in Towney, and, like Sperling J in Roach v Page, recognised that the implications were not limited to expert witnesses (at 367):
Section 126 does, however, have an incidental significance to this matter. Counsel for the applicant expressed in argument the concern that if the two documents called for are no longer the subject of the client legal privilege in the circumstances, there will be great difficulty in knowing 'where the line is to be drawn'. The spectre of waiver with respect to drafts, notes of discussions and other incidental materials was raised, and not simply with respect to expert witnesses but to any witness. Part of the answer to that concern may lie in the proper application of s 126 in the circumstances of any particular case.
45 In my view, what Enderby J said in Crawford v Bailey is no longer the law, if it ever was. What his Honour said is inconsistent with Dingwall and with Tirango. Moreover, it is not the test enunciated by Evidence Act, s 126. In my opinion, service and tender of an expert witness' report in proceedings does not constitute a waiver of the privilege which attaches to communications between the expert and the solicitors who instructed him or her, save to the extent that those communications are associated documents reasonably necessary to an understanding of the report. "Proper understanding" of a document or communication will sometimes, but not always require that documents to which it responds or refers be available. It may very likely be so when the primary document contains a summary or excerpt from an earlier communication, or responds to questions which are not themselves restated in it. But I do not accept that "a proper understanding of the communication or document" involves an appreciation of the manner in which the opinions contained in the document have been formed over time, or the iterations and evolutions through which they have passed. The test is concerned with the comprehensibility of the primary communication or document: if it can be completely or thoroughly understood without more, then access to the related communications or documents is not reasonably necessary.