What is the legal test for production of subpoena material?
When a subpoena is file and served on a person, production of material sought by the subpoena can be objected to on several grounds (see eg. UCPR 2005, Reg 33.4 and Criminal Procedure Act 1986, s 227). A common ground for objection is that the subpoena is merely a "fishing" expedition.
The test for whether a subpoena can be set aside on such a basis was confirmed by the NSW Criminal Court of Appeal in Attorney General for New South Wales v Chidgey  NSWCCA 65.
In that case Beazley JA confirmed the test stated by Simpson J in Regina v Saleam  NSWCAA 86 that "the applicant must (i) identify a legitimate forensic purpose for which access is sought, and (ii) establish that it was 'on the cards' that the documents will materially assist his case."
Accordingly the test is a two stage process. As to "legitimate forensic purpose", "mere relevance" is insufficient. The term 'on the cards' in this context arises from the judgment of Gibbs CJ in Alister v R  HCA 45 where he says "Although a mere 'fishing' expedition can never be allowed, it may be enough that it appears to be 'on the cards' that the documents will materially assist the defence."