A witness who gives the court an opinion may have no
better information than that already available to the court, and may be in no
better position to come to an opinion than the fact-finder.
In such a case the admissibility of the witness’s
opinion is (in New Zealand) governed by ss 23 – 25 of the Evidence
Act 2006. Evidence of an opinion is not admissible in a criminal case except
pursuant to ss 24 and 25.
Section 24 provides:
A witness may state an opinion in evidence
in a proceeding if that opinion is necessary to enable the witness to
communicate, or the fact-finder to understand, what the witness saw, heard, or
otherwise perceived.
Where the fact-finder is in just as good a position as
the witness to form an opinion, the s 24 requirement of necessity is not met.
Section 24 does not, in such a case, allow the opinion to be given in evidence.
Sometimes a witness will have specialised knowledge,
not available to the fact-finder, on which an opinion is based. Then the
admissibility of the opinion will be governed by s 25. Subsection (1) provides:
An opinion by an expert that is part of
expert evidence offered in a proceeding is admissible if the fact-finder is
likely to obtain substantial help from the opinion in understanding other
evidence in the proceeding or in ascertaining any fact that is of consequence
to the determination of the proceeding.
The definitions of opinion, expert, and expert opinion
in s 4 are explanatory and consistent with ordinary usage.
Merely satisfying the criteria in ss 23 or 24 does not
render opinion evidence necessarily admissible, as it may be excluded by
another provision of the Act, such as s 8 (prejudicial effect exceeding probative
value).
In a recent decision, currently suppressed ([2017]
NZCA 430), our Court of Appeal has held that another section that may apply to
exclude opinion evidence that does not go further than the fact-finder can go
without the witness’s opinion, is s 7. This is headed “Fundamental principle
that all relevant evidence admissible”, and reads:
(1) All relevant evidence is admissible in a
proceeding except evidence that is—
(a)
inadmissible under this Act or any other Act; or
(b)
excluded under this Act or any other Act.
(2) Evidence that is not relevant is not
admissible in a proceeding.
(3) Evidence is relevant in a proceeding if
it has a tendency to prove or disprove anything that is of consequence to the
determination of the proceeding.
In effect, the Court of Appeal has read words into
subsection (3), as if it read (adding the blue words):
(3) Evidence is relevant in a proceeding if
it has a tendency to prove or disprove anything and
is capable of assisting the fact-finder in determining anything that is of consequence to the determination of the proceeding.
The Court drew that from dicta requiring evidence
to be reasonably capable of supporting
the fact in issue: Bain v R [2009] NZSC 16 at
[40].
I do not think that it was necessary for the Court of
Appeal to read words into s 7. An opinion, as defined, on a fact in issue will
necessarily be relevant, unless it is equally consistent with proof, as with
disproof, of the fact in issue. It will have a tendency to prove or disprove
something of consequence to the determination of the proceeding, just as the
fact-finder’s opinion will. The point is that, in the case that has given rise
to this discussion, the witness’s opinion had no more probative value than the
opinion that the fact-finder could come to independently. It was relevant, but
inadmissible because it did not satisfy the requirements of ss 24 or 25: it was
not “necessary” and it was not likely to give the fact-finder “substantial help”.
