Reverse onus provisions for establishing a defence can conflict with the right to be presumed innocent until proven guilty, as was seen in Hansen v R  NZSC 7 (blogged here 20 February 2007).
That case drew Parliament's attention to s 6(6) Misuse of Drugs Act 1975[NZ], and the words "presumed until the contrary is proved" were held to put the burden of proof to the standard of the balance of probabilities on the accused.
It was not possible, held the Court in Hansen, to interpret this phrase as meaning until the accused raises a reasonable doubt about whether he or she had the proscribed purpose.
No doubt the New Zealand Law Commission, which is currently reviewing the legislation concerning serious drug offending, will be looking around for a way to formulate the statutory defence of absence of purpose of supply without raising a conflict with the New Zealand Bill of Rights Act 1990. That is, assuming that the possession for supply offences remain: an alternative would be to replace them with offences of possession of traffickable quantities of drugs.
Legislation that provides a potentially useful analogy was considered yesterday by the House of Lords in R v G  UKHL 13 (4 March 2009). The offences created by sections 57 and 58 of the Terrorism Act 2000[UK] were analysed here, with reference to how the defences in s 118 applied to them.
The offences involve possession, so there is an interesting discussion of what possession is in this context: see paras 46 – 48, 50, 53, and 60 – 62. In particular, the ingredient of knowledge of the nature of the information in one's possession, for the s 58 offence, was held to be akin to that required in the famous old case Sweet v Parsley  AC 132 (HL).
Sections 57(2) and 58(3) of the Terrorism Act 2000[UK] create defences if the person charged proves certain things. For what "proves" means, reference must be made to s 118(2):
"If the person adduces evidence which is sufficient to raise an issue with respect to the matter the court or jury shall assume that the defence is satisfied unless the prosecution proves beyond reasonable doubt that it is not."
And s 118(4):
"(4) If evidence is adduced which is sufficient to raise an issue with respect to the matter mentioned in subsection (3)(a) or (b) the court shall treat it as proved unless the prosecution disproves it beyond a reasonable doubt."
This is statutory recognition of the idea that the raising of an issue can amount to proof. The evidential burden is treated as a legal burden. In Hansen the Supreme Court held that "proves" cannot mean "raises a reasonable doubt", for reasons too mysterious to discern. Actually, that’s a bit dismissive. The Court held that this meaning was not available because of Parliament’s intent in 1975, because of there being only two recognised standards of proof, because raising an issue is just testing the proposition, not proving anything, and because the expression “the contrary” in the phrase “until the contrary is proved” indicates that more than merely testing the proposition is required. In effect the Court was not prepared to revise the statute by reading in a provision like s 118, above.