Criminal trials and hearings can be complex and lengthy, involving the presentation of a wide range of evidence and the examination of numerous witnesses, including experts from various fields.
While s 76(1) of the Evidence Act 1995 (NSW) provides that opinion-based evidence is generally inadmissible, the statute also outlines some exceptions, one of which permits opinions based on ‘specialised knowledge’. However, even though it provides this exception, s 79(1) does require that the specialised opinion be substantially based on the expert’s ‘training, study or experience’.
Sometimes there are doubts about whether the field of expertise is a recognisable field for judicial purposes. The test, as determined by the Australian High Court in HG v The Queen (1999) 197 CLR 414, , is whether the expert’s knowledge and experience is in an area that is ‘sufficiently organised or recognised to be accepted as a reliable body of knowledge or experience’.
As for the credibility of the expert, it was suggested in Makita (Australia) Pty Limited v Sprowles (2001) 52 NSWLR 705 that the court must be supplied with ‘criteria enabling evaluation of the validity of the expert's conclusions’ (at ). Expert evidence in criminal proceedings could include, for example, opinions regarding the sequence of events in a victim’s murder, the mechanical condition of a vehicle involved in a crime, the psychological wellbeing of the accused and/or victim, the physical and psychological effects of various drugs and so on.
Although the opinions of experts must relate to admissible facts, where there is doubt concerning the facts on which the expert’s opinion is based, the opinion itself may carry little weight or be deemed irrelevant. Contradictory opinions concerning evidence can be particularly confusing and can also facilitate convictions being overturned, as was the case in Wood v R  NSWCCA 21 (24 February 2012). At trial, experts in this case presented various opinions on whether the deceased could have jumped to her death from the gap at Watson’s Bay or whether the accused could have thrown her off the cliff. After conducting a series of experiments, Associate Professor Cross was satisfied that a strong and fit man could have thrown the deceased from the northern ledge, using a spear-throw technique, having her land on the particular rocks on which she was found (at ). Associate Professor Cross also concluded that the deceased could not have jumped to these particular rocks herself. However, another expert, Professor Pandy, opined that the victimcould have landed on the rocks in question if she had jumped from the Northern ledge (at ). The appeal court found that given the conflicting opinion and the state of the evidence, the dispute on this particular issue could not be definitively resolved (at ).
Notably, some concern exists in the legal community regarding the provision of expert evidence, including the cost of attaining the evidence and the delay it can cause. Concern also exists about the impartiality of the experts that are relied on. These concerns are not limited to criminal trials but also relate to civil proceedings. While the government implemented many of the recommendations made by the Australian Law Reform Commission in its 2006 report on the uniform evidence law, concern remains regarding procedures related to the provision of expert opinion (Honeysett v The Queen  HCA 29 (13 August 2014)).
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