Witnesses — not called
No comment should be made as to the failure of the defence to call a witness who might have been able to assist the defence: Dyers v The Queen (2002) 210 CLR 285. If any comment is to be given it is that the jury should not speculate about what a witness not called might have said: Dyers at .
In Mahmood v Western Australia (2008) 232 CLR 397 at  the High Court held that in a criminal trial:
… where a witness, who might have been expected to be called and to give evidence on a matter, is not called by the prosecution, the question is not whether the jury may properly reach conclusions about issues of fact but whether, in the circumstances, they should entertain a reasonable doubt about the guilt of the accused.
See also Louizos v R, R v Louizos (2009) 194 A Crim R 223 at .
[4-375] Suggested comment — witness not called by prosecution
You have heard that [name of witness] has not been called by the Crown to give evidence. You can take the fact that there was no evidence from that witness into account when you decide whether the Crown has proved the guilt of the accused.
I am not inviting you to guess what [name of witness] would have said if [he/she] had been called. You must not do that at all. But in a criminal trial, where the Crown must prove that the accused is guilty beyond reasonable doubt, a jury is entitled to take into account that there was no evidence from a particular person in deciding whether or not there is a reasonable doubt about the accused’s guilt … [refer to the submissions of the defence and Crown on the issue].
[4-377] Suggested direction — complainant not called on retrial
The appropriate direction to be given where a complainant did not give evidence in person in accordance with s 306B Criminal Procedure Act 1986 was considered in PGM (No 2) v R  NSWCCA 261 at –.
It must be obvious to you that [the complainant] did not personally give evidence before you. Instead a [video-tape and/or audio record] of the complainant’s evidence from an earlier trial was played to you. The procedure that was adopted in this trial is usual practice. It is to spare [the complainant] from being required to give evidence in person more than once.
You cannot use the fact that [his/her] evidence was played to you from a [video tape or audio recording of his/her evidence] against the accused. You should not give the evidence any greater or lesser weight simply because it was given in this way. You should assess the evidence in the same way as you assess the evidence of any other witness.
It will also be obvious to you that the complainant has on another occasion been cross-examined by counsel on the evidence relied upon by the prosecution in this case. You should not speculate about the reason why the complainant’s evidence had to be given again before you. There are a number of reasons why a witness may be required to give evidence before a court more than once on the same subject matter. And there is a procedure that has been adopted here to permit the evidence to be given without the witness being required to attend court.
You cannot speculate about what [the complainant] may have said had [he/she] been called to this trial. You simply act upon the evidence that is before you and assess it to determine whether you are prepared to act upon it beyond reasonable doubt. You may feel that there were areas of the evidence that could have been further explored by either party, but you cannot speculate about such a matter or what any further exploration may have revealed, if anything. You must simply consider the evidence that you have.
Of course you may take into account that you have not seen the complainant personally. It may be that the witness’s demeanour or behaviour while giving evidence, particularly during cross-examination, may have been able to assist you in determining whether you accept the evidence of the witness beyond reasonable doubt. Whether in some way the absence of the witness deprives you of some assistance in evaluating the evidence, is really a matter of speculation. If however you feel in some way that you have been deprived of that assistance then that is a matter you take into account when determining whether the Crown has proved its case beyond reasonable doubt.