What will be the strength of NSW decisions on their Evidence Act 1995 (which is not identical with our new Act) in Victorian courts? Will those cases be binding or merely persuasive?
It’s impossible to argue that they won’t, at the least, be highly persuasive. There’s no point re-inventing the wheel. The decisions of other jurisdictions (and, to a lesser extent, the various reports that preceded the creation of the UEA) will be referred to frequently in legal argument. But will judges and magistrates be obliged to follow interstate precedents, or just be assisted by them?
The Laws of Australia
There’s judicial authority that suggests, in most cases, that they must follow. In Australian Securities Commission v Marlborough Gold Mines (1993) 177 CLR 485 the High Court spoke with one voice in saying [at 4],
…[U]niformity of decision in the interpretation of uniform national legislation such as the Law is a sufficiently important consideration to require that an intermediate appellate court – and all the more so a single judge – should not depart from an interpretation placed on such legislation by another Australian intermediate appellate court unless convinced that that interpretation is plainly wrong.
This extends national uniformity beyond mere judicial comity to a binding legal principle. It’s been applied in a variety of contexts: corporations law (Powell & Duncan v Fryer & Anor  SASC 59) disclosure (Workcover NSW v Law Society NSW  NSWCA 84), property (Anderson v Wilson  FCA 394) and motor traffic law (DPP v Moore (2003) 6 VR 430), just to name a few. There is only one common law in Australia: Lipohar v R (1999) 200 CLR 485.
This point about both statutory interpretation and the common law was made even more expressly in the High Court’s decision in Farah Constructions Pty Ltd v Say-Dee Pty Ltd (2007) 230 CLR 89 [at 135],
Intermediate appellate courts and trial judges in Australia should not depart from decisions in intermediate appellate courts in another jurisdiction on the interpretation of Commonwealth legislation or uniform national legislation unless they are convinced that the interpretation is plainly wrong. Since there is a common law of Australia rather than of each Australian jurisdiction, the same principle applies in relation to non-statutory law.
(French CJ had not yet taken up his appointment in the High Court when Farah was handed down. He recently approved this view in extra-judicial remarks in his 2009 State of the Judicature address.)
Apples and Oranges?
This doesn’t mean that NSW and Victoria will become the legal equivalent of conjoined twins, though. I’ve said elsewhere that there are traps involved in blindly relying on Commonwealth or NSW authorities to resolve questions of evidentiary law in this state.
First, because nationally uniform legislation always starts off with the intent of unifying legislation, but it rarely stays that way. The Road Rules are a classic example of this. Often the differences in the various Evidence Acts can be subtle, but important.
The second reason interstate decisions can only be of limited application to our law can be illustrated with a simple example. Say a submission is being made that a court ought to exercise its discretion under s 138 to refuse to admit evidence in the exercise of judicial discretion, cases like R v Fleming  NSWCCA 233, R v Naa  NSWCA 851 or R v Helmhoudt  NSWCCA 372 may assist with understanding how the court might exercise its discretion. But it would be a mistake to rely on the bare facts of any of these cases and attempt to draw non-existent parallels between the NSW cases and the matter under consideration, for the obvious reason that what might be considered unlawful in NSW may be lawful in Victoria, and vice versa.
It’s arguable that what constitutes improper conduct for the purposes of s 138 can be established to a national uniform standard. But it won’t be possible to determine what is lawful or unlawful without reference to the particular laws of the jurisdiction. There will often be these kinds of threshold issues that need to be resolved before the proper application of the Evidence Act even falls to be considered.
No principle of law or comity requires that the distinct statutes of each state be drawn together so that they can be treated as identical, when they are not. It follows that care needs to be taken when comparing factual scenarios from north of the border.