It’s not uncommon for a suspect to disclose their involvement in an offence to investigators “off the record”, but protest their innocence or make no comment when formally interviewed.
Why someone would do this is difficult to understand. It would appear far more logical for a person to approach their dealings with police with a particular strategy in mind – either to answer all questions truthfully, or to make no comment, or to present a particular version of events – and stick to that strategy throughout. But there are a sufficient number of recorded cases to confirm that these conversations do happen.
Woon v R (1964) 109 CLR 529 is an example of a case where the accused remained undecided which strategy to adopt during the formal interview itself, changing his mind repeatedly and ending up looking guilty and evasive as a result. In R v Dupas  VSCA 109 the accused declined to repeat in the interview what he had said while being driven to a police station.
In Em v The Queen (2007) 232 CLR 67, the accused had indicated that he would make no comment to any questioning that was tape recorded. Several weeks after the formal recorded interview where he made no comment, police were granted warrants to secretly tape-record conversations with the accused. He knew he was talking to police and was advised that what he said could be used in evidence, but wasn’t told he was being recorded. The Supreme Court allowed part of this recording to be admitted, a decision the High Court later upheld.
The High Court had earlier decided in R v Swaffield; Pavic v The Queen (1998)192 CLR 159 that courts have a broad discretion to reject evidence on the ground that unfair prejudice would result from its admission into evidence. Likewise, the effect of the impugned conduct of police is one of a number of factors to be considered in deciding whether judicial discretion to refuse to admit evidence will be exercised (though the undesirability of the conduct itself is the subject of separate consideration under the public policy discretion). In Pavic‘s case, the trial judge admitted the evidence. The High Court upheld that decision.
The case of Carr v Western Australia (2007) 232 CLR 107 is a recent variation on this theme.
The accused denied involvment in an armed robbery when interviewed. However, when later discussing the robbery with police in a holding area of the police station, he said things that clearly implicated himself. The accused was unaware that the area was video- and audio-recorded when he volunteered these statements. He was convicted, and while not disputing the truth of the admissions he made, it was argued on appeal that he made the statements implicating himself to, “frustrate”, “tease” and “piss off” the investigators, not expecting the evidence would be used against him.
The High Court rejected the notion that an accused must consent to being videotaped before admissions could be considered voluntary.
Although the Court’s reasoning centred around the interpretatation of Western Australian legislation (WA hasn’t yet adopted a Uniform Evidence Act), parellels can be drawn between WA laws and those in Victoria. As Keith Chapple SC noted in his summary of the case in the NSW Bar News:
The whole circumstances of the case are a cautionary tale for any counsel offering advice to a suspect who is ‘assisting with enquiries.’ If a client is exercising a right to silence it should be constant when in the company of the police.