Enforcement officers will sometimes need to seek the opinions of experts or suitably qualified professionals to establish whether an element of an offence has been met. This could happen when the evidence is of a particularly technical nature, requires legal interpretation, or requires the input of someone with extensive experience of a matter that is difficult to objectively measure.
On other occasions, witnesses may be keen to make comments about the matter under investigation, offering their opinion on particular facts when making a statement.
Generally, a Court will not admit opinion evidence unless it is from an expert on the facts involved and will offer substantial assistance to the fact-finder judge or jury (refer s23-25 of the EA which relates to statement of opinion and expert evidence). The opinion rule is important to consider when:
finding appropriate witnesses for the case
preparing briefs of evidence from statements given by witnesses.
Note however that s24 of the EA provides that a witness may state an opinion in evidence if that opinion is necessary to enable him or her to communicate, or the fact-finder to understand, what the witness saw, heard, or otherwise perceived.
Veracity and propensity evidence
During the course of an investigation, the enforcement officer may well find out information about activities, relationships and other dealings of a suspect that indicate they are not truthful (veracity) and /or that they are likely to have committed the alleged offence (propensity).
These are matters of character. Evidence about them is strictly controlled by the Courts, because they may prejudice the fact finder (especially a jury) even if they are not directly relevant to the case.
Veracity evidence is not admissible unless it is substantially helpful in assessing a person's truthfulness. Relevant examples under s37(3) of the EA include:
the person's disposition to tell the truth when under a legal obligation to do so (for example, in an earlier proceeding or in a signed declaration)
previous convictions for offences of dishonesty
previous inconsistent statements
a motive to be untruthful.
The prosecution may not offer veracity evidence about the defendant unless the judge gives leave and the defendant first raises veracity, either by giving such evidence about him or herself or by challenging the veracity of a prosecution witness by reference to a matter other than facts in issue.
Propensity evidence is typically encountered when enforcement officers check records to see whether a suspect has been the subject of previous investigations.
The prosecution may only offer propensity evidence about a defendant in accordance with s43 of the EA. The judge has discretion to allow propensity evidence by balancing its value as evidence against the risk of an unfair prejudicial effect. For example, previous convictions for similar offences are typically very prejudicial.
Whether the effect of allowing propensity evidence is unfair depends on:
the extent of any similarity between the offending acts or omissions
the extent to which the acts are unusual (i.e., no ordinary person would do this repeatedly)
the level of repetition, and the lapse of time between offences.
If offering evidence of previous convictions, the prosecution must first inform the judge as to the purpose of doing so.