If two defendants, A and B, are on trial together, when can A rely on part of B's statement to the police if B does not give evidence?
Broadly, once evidence is admissible it can be used for all purposes at trial unless there is a specific restriction: Hart v R  NZSC 91 at . Does s 27 of the Evidence Act 2006 provide for a restriction? Its first subsection says:
"Evidence offered by the prosecution in a criminal proceeding of a statement made by a defendant is admissible against that defendant, but not against a co-defendant in the proceeding."
Does this prevent the defence "offering" it? ("Offer evidence" is defined inclusively and widely in s 4.) Plainly no, although this has been controversial. Difficulties have concerned the meaning of "against". The answer seems to be that which is given in Leslie-Whitu v R HC Rotorua CRI-2009-263-163, 5 October 2011, Woolford J (not available online): s 27 does not prevent A relying on B's statement, but the statement is hearsay in relation to A. Since the evidence in relation to A must be considered separately from that in relation to B in a joint trial, A must rely on the hearsay provisions to get B's statement admissible in his case. As hearsay the rule is that it would be inadmissible, but there are exceptions, the main one of which is s 18 of the Evidence Act 2006.
The criterion in s 18 when B is unavailable, as he would be if he chose not to give evidence, is that "the circumstances relating to the statement provide reasonable assurance that the statement is reliable". The "circumstances" are defined in s 16.
I don't know whether that is the best answer. It creates an uneven playing field in that to use B's statement A must overcome the "reasonable assurance" of reliability threshold in s 18, whereas the usual requirement for admissibility is relevance, s 7, which has a low threshold: Wi v R  NZSC 121, mentioned here on 11 August 2012. If the prosecution can get the statement admitted under s 27 (overcoming any objections based on unreliability, s 28, oppression, s 29, and impropriety, s 30) then A should be able to use it too without further restriction.
I suggested in a New Zealand Law Society Seminar that arguably the admissibility of B's statement for A's case should be governed by the common law fairness considerations.
Yesterday the High Court of Australia touched upon this in a common law context: Baker v The Queen  HCA 27 (15 August 2012). However here B's statement did not really exculpate A: see Heydon J at -. But basically the Court said, "Aw fieck orf moit" (I assume Australians all sound like Hughsey).
The case illustrates how the common law has diverged in various jurisdictions. Canada and England (see Baker at , ) have expanded the hearsay exceptions to permit use of B's statement by A if it is apparently reliable, but as the judgments in Baker show, there are difficulties in establishing reliability in this context.
I still think use of the hearsay exception here is inappropriate because it is only by chance that B's statement is hearsay – at least under our statutory definition of hearsay which does not include within it statements made previously by a witness (see the definition of "hearsay statement" in s 4). If B gives evidence he is a witness so it is not hearsay, whereas if he does not give evidence it is. Should the trial tactics of one party govern the admissibility of evidence for another? And if the prosecution can rely on B's exculpation of A and argue it is a lie demonstrating B's guile, why can't A rely on it and argue it is a glimpse of B's truthfulness?
The common law position is different because the definition of hearsay is not dependent on whether the maker of the statement, B, gives evidence. Baker remains of interest for its emphasis on the difficulties in applying a criterion of reliability.