Ultimate Issue Questions:
When preparing a letter of instruction for an expert witness, it is recommended that legal practitioners abstain from directly asking experts ‘ultimate issue’ legal questions. This is due to the risk that the expert witness’s answer will be deemed as being beyond their expert knowledge and be inadmissible, by virtue of not falling under the Expert Exception Rule in Section 79 of the Evidence Act.
The Ultimate Issue Rule Abolished
The Ultimate Issue Rule is the common law rule that prevented expert witnesses being asked ultimate issue questions. This rule was abolished by the implementation of Section 80 of the Uniform Evidence Acts, in part due to widespread criticisms of the rule as “uncertain, arbitrary in its implementation, and conceptually problematic”.  Because of this, ultimate issue questions can technically be asked of experts. Indeed, in some areas such as property valuations, experts frequently answer questions that go directly to the ultimate issue. This is because their area of expertise falls within the legal issue at hand. In most other areas, this will be unlikely to be the case and should therefore be avoided.
Rephrasing the Question:
This potential problem can often be easily avoided through rephrasing, to ensure that the question asked clearly falls within expert knowledge. For instance, in a matter concerning professional negligence, rather than asking an expert witness “Was the defendant negligent?” a better rephrasing of that question would be “Did the defendant act in a manner that was widely accepted in Australia by peer professional opinion as competent professional practice?”
Similarly, if the question was on the subject of testamentary capacity, rather than asking “Did the testator possess testamentary capacity?”, it would be preferable to ask, “Could the acquired brain injury suffered by David have any cognitive effect on him that may have resulted in David making an unwanted disposition in his estate?”. The best rephrasing obtains the same result while ensuring the question remains within the expert’s field of specialised knowledge
Most expert witnesses do not have direct legal expertise and therefore an expert witness answering a legal question risks being beyond the expert’s purview. Through the rephrasing of questions to focus on an expert’s area of expertise, legal practitioners will still be able to obtain answers on the issues pertinent to the case at hand, while ensuring that their expert witness’s comment falls with the Section 79 exception of the Evidence Act.
 The Opinion Rule and its Exceptions [https://www.alrc.gov.au/publications/9.%20The%20Opinion%20Rule%20and%20its%20Exceptions/opinion-ultimate-issue]