From January 1 2010, under s 139 of the Evidence Act 2008 a caution against self-incriminination will need to be given to every suspect who an investigating offical believes has committed an offence. If not given, any subsequent admissions may be excluded under s 138 as having been improperly obtained.
That’s a change from the current position, where an investigating official must typically possess a power of arrest for the suspected offence before a warning against self-incrimination is required.
The practical effect of this will be to encourage the police to caution against self-incrimination when investigating summary offences.
Most of the time it will be quite clear when such a belief was present. In other cases it will not be obvious if investigators believed an offence had been committed. Suspicion — a less certain state of mind than belief — is insufficient to require a caution: R v Pearce  NSWCCA 447.
Under s 138(1),
(1) Evidence that was obtained—
(a) improperly or in contravention of an Australian law; or
(b) in consequence of an impropriety or of a contravention of an Australian law—
is not to be admitted unless the desirability of admitting the evidence outweighs the undesirability of admitting evidence that has been obtained in the way in which the evidence was obtained.
There then follows a number of considerations for a court deciding this question. Those factors are similar to those in the High Court’s decision in Bunning v Cross.
As I discussed here recently, the consequences of unlawful or improper conduct by police are often not dire for the prosecution. They often don’t even result in the exclusion of the evidence that the impropriety facilitated.
Though frequently invoked, there’s now a substantial number of NSW cases where appellate courts have upheld discretion exercised to allow improperly or unlawfully obtained evidence to be admitted. (Though it’s important to recognise that not being excluded by s 138 doesn’t mean that evidence can’t be excluded under another of the discretions.)
R v Naa  NSWSC 851 is an example. There, NSW police members were tensely negotiating with a man, trying to persuade him to disarm so medical treatment could be provided to his injured (it later became apparent, deceased) partner. The statements made by the accused were inculpatory, and he declined to repeat them when later questioned in a formal interview.
On trial for murder, the accused challenged the admissibility of these inculpatory statements, made without benefit of caution. After discussing the relevant NSW provisions about recording admissions (analagous to custody provisions in our Crimes Act), Howie J turned to non-compliance with 139 of the Evidence Act 1995 (NSW):
 I do not believe that the conversation between officer McCarthy and the accused amounted to “questioning” for the purpose of s 139. The questions asked by McCarthy were completely incidental to what she was trying to achieve: to convince the accused to put down the weapons for the safety of himself and others. She was not seeking information from him. She was seeking to distract him, to mollify him and ultimately to persuade him to disarm. No bystander watching the incident would have considered that the officer was questioning the accused. The section was never intended to apply to a siege situation where the police were involved in negotiation rather than investigation.
 In any event, even if I were wrong in that view, it does not follow that the absence of a caution means that the evidence should be rejected. Section 138 applies and I am required to undertake a balancing exercise to determine whether “the desirability of admitting the evidence outweighs the undesirability of admitting evidence that has been obtained in the way in which the evidence was obtained”. In determining that balance I am required to consider relevant matters including those mentioned in s 138(3).
 In the present case the substantial majority of those considerations is very strongly in favour of admitting the evidence. There is no doubt that the admissions made by the accused at the scene are highly probative on the charge of murder and very important in the proceedings. They impact significantly upon the two issues that have been identified: intention and substantial impairment. The offence is clearly of very great gravity.
 Such contravention as there was by the police in failing to administer a caution is in my opinion relatively minor having regard to the particular facts of this case. The contravention was not intentional: McCarthy never thought of administering a caution because she did not believe that she was questioning the accused in an attempt to obtain admissions. Inspector Winmill believed that a caution would have been inappropriate because it was important that the accused continue to communicate with the police. The contravention was not inconsistent with the rights of the accused under the International Covenant on Civil and Political Rights: see R v Em  NSWCCA 374 at .
 There will be no other proceedings taken in respect of the contravention. The difficulty of obtaining the evidence without contravention does not seem to me to be a relevant consideration.
 The decision whether to admit evidence, notwithstanding that it was obtained improperly, is based upon public policy considerations being weighed against one another. In my opinion the weight of those considerations falls very substantially upon the desirability of admitting the evidence. I do not believe that any other view is reasonably open.
It’s likely that impropriety of various kinds by investigating officials will be identified through the application of the new Evidence Act. Whether that leads to the exclusion of the evidence thus acquired is a separate question.