"It is always within the legal power of the jury on a charge of
murder to return what the High Court has called the 'merciful
verdict of manslaughter'; Hughes v. The King, and it is a
misdirection if words are used which deny them that power. The
rationale of the rule is explained by the High Court if Beavan v
The Queen. See also Brown v The King; Packett v The King; Mraz
v The Queen. In some of these cases the court was considering the
provisions of statutes codifying the criminal law, but the
doctrine in question applies equally at common law. See per Dixon
J, in Packett v The King; R v Ryan and Walker.
It is, of course, in no way inconsistent with this doctrine that
the judge, if he thinks there is no evidence on which a reasonable
jury could find a verdict of manslaughter, is entitled so to tell
them or to refrain from saying anything about manslaughter at all
(Beavan v The Queen). And indeed it may be a misdirection if he
invites them to consider the question of manslaughter as
reasonably open when the only verdicts which a reasonable jury
could find are guilty of murder or not guilty (Mraz v The Queen).
It must be admitted that the task of a judge under these
circumstances is no easy one. If there is no evidence which can
reasonably justify a verdict of manslaughter, he must either, if
the question is raised, tell the jury so or be silent on the
topic: at the same time he must not in terms deny their right to
return a verdict of manslaughter if they want to do so in defiance
of his direction. And, of course, if he tells them that
manslaughter is not a reasonably possible verdict on the facts and
the court of appeal thinks it is, or that it is a reasonably
possible verdict on the facts and the court of appeal thinks it is
not, the verdict is liable to be quashed on appeal.
Nevertheless if he tells the jury that they ought to return a
verdict of manslaughter, he must be careful that they are in no
danger of understanding his direction as a direction that they
cannot return such a verdict."