Taub v R [2017] NSWCCA 198

Taub v R [2017] NSWCCA 198 · Read the judgment on AustLII

Catchwords: Expert evidence – Admissibility – Assumptions upon which opinion is based need not be proved

The CCA has determined that the assumptions on which an expert opinion is based need not be proven in order for the opinion to be admissible.

The appellant was found guilty of manufacturing a large commercial quantity of methylamphetamine (at the time, the commercial quantity was 1kg – it is now less). In his garage, police found a modified beer keg which contained more than 27.6kg of material which was seemingly a by-product of methylamphetamine production. Police also found buckets containing about 7.8kg and 9.1kg respectively of what were said to be substances related to drug manufacture. A forensic chemist gave evidence about the substances – and, crucially, given the amounts of those by-products, the amount of drug which might have been manufactured. In the way that the appeal ultimately played out, the only issue was whether the amount manufactured was over 1kg.

During his evidence, the forensic chemist made certain assumptions about the by-products. One question on the appeal (the only issue this summary deals with) is whether those assumptions needed to be proved in order for the opinion to be admissible.

Simpson JA (Walton and Button JJ agreeing) allowed the appeal on the basis that, although methylamphetamine was manufactured, the expert opinion allowed for the possibility that the amount was less than 1kg. However, the court simply substituted a conviction for a lesser offence of manufacture, and remitted the matter to the District Court for sentencing.

The tests for admissibility of expert opinion are commonly attributed to two separate opinions of Justice Heydon, in two separate courts: Makita (Australia) Pty Ltd v Sprowles (2001) 52 NSWLR 705 (as Heydon JA), and Dasreef Pty Ltd v Hawchar (2011) 243 CLR 588 (as Heydon J – now of the High Court). One question which arises is: must the basic assumptions upon which an expert opinion is based be proved in order for the whole opinion to be admissible?

After close analysis of the decisions (at [21] to [33]), Simpson JA concludes that proof of the truth of the assumptions is not a condition of admissibility. There remains a small question about the rigour on which a court should insist in relation to the availability of evidence to prove the truth of the assumptions (see [31] and [32]). But it seems that, provided the facts and reasoning are adequately disclosed, the opinion is admissible (even if subsequently found to be flawed or wrong) – if the assumptions are wrong, then that goes to the weight to be given to the evidence (but not usually its admissibility).

In a practical sense, the consequence for practitioners is that it is still vital to test (on the voir dire, if necessary) the assumptions upon which an expert opinion is based – but it will not be sufficient to defeat admissibility simply to show that the assumptions cannot be proven to be correct.