In SPA the Crown sought the admission of evidence of charged and uncharged sexual acts against the victims, who were family members of the accused. The accused denied the charges, and it was uncontentious that the tendency and coincidence evidence of other acts was relevant. The accused asserted at trial and on the appeal that the evidence was insufficiently probative. Neither court agreed.
A late ground for challenging the admissibility of the evidence was the possibility of concoction. Before the introduction of s 398A of the Crimes Act (repealed when the Evidence Act 2008 came in), and still in some other jurisdictions, the possibility of concoction (witnesses agreeing on a story) needed to be rebutted (in some cases, excluded) before similar fact evidence could be admitted. The extract of the interlocutory judgment referring to concoction [Hansen JA at 11, Weinberg JA and AJA Judd agreeing] is brief:
The applicant raised one further point that was not raised before the judge. That was that concoction or contamination may be considered when considering the probative value of the evidence. Doubtless in principle that is so, but there must be a basis for doing so as distinct from mere speculative suggestion. If it is desired to raise the point it should be done before the judge. To seek to raise the point before this court now for the first time is to short-circuit the ordinary processes. It is apparent from what counsel said that it is a point that he desires to raise and accordingly it should be left to be considered by the trial judge. In that sense the present application may be seen to be somewhat premature.
The fleeting reference affirms the approach taken in NSW under the UEA. As the NSW Court of Appeal said in BP v R; R v BP  NSWCCA 303, [Hodgson JA at 108, Price and Fullarton JJ in agreement],
It is not necessary in criminal cases that the incidents relied on as evidence of the tendency be closely similar to the circumstances of the alleged offence, or that the tendency be a tendency to act in a way (or have a state of mind) that is closely similar to the act or state of mind alleged against the accused; or that there be a striking pattern of similarity between the incidents relied on and what is alleged against the accused: Ford at , , PWD at -. However, generally the closer and more particular the similarities, the more likely it is that the evidence will have significant probative value.
The possibility of prejudicial effect with which s 101 is concerned is the possibility that the jury will act on the evidence otherwise than by way of its rational effect on the probability of a fact in issue, for example by giving effect to “some irrational, emotional or illogical response” or “giving the evidence more weight than it truly deserves”: R v Suteski (2002) 56 NSWLR 182 at . An assessment must be made whether the probative value of the evidence substantially outweighs any prejudicial effect that the evidence may have: R v Ellis (2003) 58 NSWLR 700 at - . If the evidence passes the s 101 test, it will a fortiori not be excluded under s 137: Ford at .
One matter that powerfully affects both the probative value of tendency evidence and the possibility of prejudicial effect is the risk of concoction or contamination of evidence. If the evidence of tendency from different witnesses is reasonably capable of explanation on the basis of concoction, then it will not have the necessary probative value: Hoch v The Queen (1988) 165 CLR 292. However, this will be so only if there is a real chance rather than a merely speculative chance of concoction: R v Colby  NSWCCA 261 at , R v OGD (No 2) (2000) 50 NSWLR 433 at , . The onus is on the Crown to negate the “real chance” of concoction: OGD at , R v F (2002) 129 A Crim R 126 at .
Relevant to consideration of concoction are the factors mentioned in Hoch at 297, namely relationship, opportunity and motive. One of these on its own is not sufficient to base a finding of a real possibility of concoction: R v RN  NSWCCA 413 at , OGD at  – .
The trial judge will consider the evidence from this fresh perspective, if it's raised. In order for the possibility of concoction to influence its probative value (assuming it can) there will need to be a foundation laid that establishes concoction may have occurred.