Australia: An instruction manual for the hot tub? New guidance from the CIArb on concurrent evidence in arbitrations

Last Updated: 30 July 2019
Article by Owain Stone

In this article, Owain Stone and Samantha Farthing discuss the new Guidelines for Witness Conferencing in International Arbitration published by the Chartered Institute of Arbitrators.

In April 2019, the Chartered Institute of Arbitrators ('CIArb') published "Guidelines for Witness Conferencing in International Arbitration" together with Explanatory Notes. 1

"The Guidelines provide a non-exhaustive checklist of factors to consider in determining a procedure that will further the efficient and effective taking of evidence and procedural orders that may be used as a basis for crafting appropriate directions for witness conferencing."

Witness conferencing, which is more commonly known in Australia as 'concurrent evidence', or more colloquially as 'hot tubbing', has become an increasingly popular form of taking expert evidence.

"Witness conferencing can be described as any evidence-taking process whereby two or more witnesses give evidence concurrently before a tribunal."

As the CIArb Guidelines explain, there is no one single approach to concurrent evidence. The manner in which it happens has many different forms, and the exact format should depend on a range of factors. The myriad different formats that can be used are part of the reason why, we consider, it has not always been a satisfactory experience – for counsel, the Tribunal or the experts. Whilst it has not always been successful, it should be remembered that the 'traditional' manner of adducing expert evidence has also been far from successful in many cases.

Concurrent evidence is one example of a technique that is increasingly being used, along with jointly appointed experts, consistent questions being considered by party appointed experts and, perhaps most importantly, joint expert conclaves and reports prior to giving evidence. It is important that all participants in the dispute resolution process proactively consider the use of these techniques to facilitate the provision of cost effective and clear expert evidence.

Expert witnesses – some terminology

Witness conferencing – usually referred to as Concurrent Evidence (more colloquially as 'hot tubbing') is the process whereby two or more witnesses give evidence at the same time

Joint expert conference – otherwise known as an expert conclave, is the process whereby two or more experts meet, normally prior to giving evidence (but after the preparation of their individual expert reports), to consider a specific question or questions. They are often asked to identify those areas on which they agree, those on which they disagree, and – perhaps most importantly – why they disagree on those points. This can often be documented by way of a joint expert report. Our experience is that this is more of a 'process' than a single event, and sufficient time should be allowed to ensure that maximum value is gained from this phase of expert evidence. Some arbitrators and judges are trying to avoid initial expert reports and are ordering a joint expert conference and joint expert report as the first step in the expert process.

Consider early – 'if not, why not?'

Whilst the CIArb Guidelines are described as 'self-evident', our experience is that experienced counsel, tribunals, experts and parties do not always spend enough time, at an early enough stage, considering the way in which expert evidence can be most effectively adduced.

We have heard counsel say that they don't want concurrent evidence, for the sole reason that the other party wanted it. However, it is not a simple decision; it is not necessarily the case that if the use of concurrent evidence may be seen as beneficial to one party in a dispute it will necessarily be a negative for the other party.

The factors identified by the CIArb Guidelines are a useful check list to help all involved assess whether it is likely that concurrent evidence will be useful for the tribunal/arbitrator and/or one or more of the parties involved.

Increasingly we are seeing some judges and arbitrators adopt a 'if not, why not' approach to concurrent evidence, meaning that the parties have to explain why they do not think concurrent evidence is appropriate, rather than trying to justify its use. Indeed, since January 2007, the NSW Supreme Court Common Law Division – General Case Management List has stated that:

All expert evidence will be given concurrently unless there is a single expert appointed or the Court grants leave for expert evidence to be given in an alternate manner." 2

Whilst there are some situations where concurrent expert evidence may not be appropriate, our experience is that its use, together with early discussion as to how expert evidence will be dealt with, can produce significant efficiencies in the time and costs associated with expert evidence, which can often be a considerable part of the costs of arbitration (or litigation).

Other reference sources

Australia is said to be the most experienced jurisdiction with concurrent evidence, as the practice was first used here, and has now been developed more than in any other jurisdiction. Over the years, a number of Law Reform Commissions have considered this approach. It has also been addressed in one of the few pieces of empirical study regarding the use of concurrent evidence and other approaches to the management of litigation, including expert evidence .

Across Australia there are also a number of specific guidelines, practice notes, procedural rules and other useful resources relating to concurrent evidence, including:

Advantages of concurrent evidence

The CIArb Guidelines provide a summary of perceived advantages of concurrent evidence, such as:

  • side-by-side presentation of evidence, by issue, makes it easier to compare differing views;
  • the ease with which experts can challenge each other's views;
  • the fact that experts may be less likely to make technically incorrect assertions in front of a peer; and
  • the process of evidence-taking is made more efficient by hearing all evidence from the witnesses on an issue at once.

From our experience, we would concur with these suggested advantages. We would stress, however, that to maximise the benefit from concurrent evidence it is preferable for an expert conclave resulting in a joint expert report to have occurred well ahead of any concurrent evidence. In this respect, we have noted that a number of judges and arbitrators have commented that the joint expert report can be one of the most useful documents in a dispute.

Issues for concurrent evidence

There are still potential downsides with concurrent evidence; the technique is not without its critics and it will not work in all circumstances. Dispute resolution has been described as a 'contact sport' and some counsel may be reluctant to relinquish control over adducing and testing expert evidence through concurrent evidence and expert conclaves. The CIArb Guidelines provide an excellent practical checklist of the matters to be considered in determining whether or not concurrent evidence may work for a particular matter, including:

Credit of one or more witness is in issue

The CIArb Guidelines identify that an issue to consider is the extent to which the credit of an expert witness is going to be subject to attack. Our experience is that there are significant challenges to attacking the credit of one or more expert witnesses if concurrent evidence is to be used. When expert witnesses are giving evidence concurrently, there is a degree of 'expertise by association', and the mechanics of concurrent evidence, particularly if it is to be dealt with on an 'issue by issue' basis (as is often most useful), make strong cross-examination on issues of credit or expertise difficult. One factor that counsel may not appreciate is that it is very difficult for an expert to be in the same witness box as one of her or his peers, whilst that other expert is being rigorously cross-examined on credit or expertise.

If there is to be a significant challenge to the credit or expertise of an expert, it may be necessary for the experts to be called separately, but one after the other (typically after the fact witnesses have all been dealt with). This approach allows a party to employ more traditional cross-examination techniques to test the credit, expertise and potential bias of an expert, whilst maintaining the advantages of the Tribunal being able to hear the issues (almost) contemporaneously – a few hours, rather than days, or potentially weeks apart and – if necessary – after all factual witnesses have been heard.

Relationship between witnesses

The concurrent evidence process may become difficult where expert witnesses are unfriendly or hostile toward each other. There is also the potential for confident, experienced and assertive experts to unfairly dominate the process, or for cultural issues or pre-existing relationships between experts to prevent one expert from asserting him/herself.

The key role of the Tribunal in successful concurrent evidence

Of the various factors identified in the CIArb Guidelines, our experience is that a key – perhaps the key – requirement for successful concurrent evidence is a proactive Tribunal. Whilst many practitioners who come from a Commonwealth law background struggle with the idea of an interventionist trier of fact, concurrent evidence lends itself to an interventionist approach from the Tribunal. At the very least, the Tribunal needs to have done its homework and be proactively involved in any concurrent evidence sessions.


The CIArb Guidelines identify a number of practical logistical issues, including such important issues as the seating arrangements, adequate physical space and the need for recording arrangements. Our experience is that these practical details are often not considered early enough, and a number of us have – with an entire court room – wasted time whilst counsel and a trier of fact discuss where we are going to sit and which microphone we will use! These issues should be dealt with before the experts are called.

Factual witnesses?

The CIArb Guidelines cover the use of concurrent evidence for factual, as well as expert witnesses. Whilst this may be possible, we have not had direct experience of this use of concurrent evidence. We can see that it may have its place, but concurrent evidence provides challenges for cross-examination, particularly about issues of credit, which is traditionally a key aspect of challenging factual evidence.


Australia was the birthplace of concurrent evidence; since it was first developed in the 1970s in the Australian Competition Tribunal, the concept – and the reality – has spread far and wide around the world – both in courts of many jurisdictions and increasingly in arbitrations. We welcome the CIArb Guidelines as a further positive step in the development of efficient and effective use of expert witnesses.

The CIArb Guidelines provide a useful aide-memoir which, together with the other resources we have identified, allow all legal practitioners, experts and arbitrators to proactively consider, at an early stage of proceedings, the potential advantages of well managed expert evidence.


Whilst there are a number of issues which should always be considered, it is important to understand that there is no 'one size fits all' for managing expert evidence. Concurrent evidence is a useful addition to the tools and techniques that can be used to cost-effectively adduce evidence from experts. However, the exact blend of these various tools and techniques, and the way in which each is to be used will differ depending on the case; the only constant is the need to consider these issues at an early stage, and revisit the approach as the case evolves.


1 Chartered Institute of Arbitrators, "Guidelines for Witness Conferencing in International Arbitration", April 2019,

2 Supreme Court of NSW, "Practice Note No. SC CL 5 - Supreme Court Common Law Division – General Case Management List", January 2007,

New South Wales Law Reform Commission, "Report 109 – Expert witnesses", June 2005,

"Managing Justice: A Review of the Federal Civil Justice System (ALRC Report 89)", February 2000,

Victorian Law Reform Commission, "Civil Justice Review: Report", May 2008,

Law Reform Commission of Western Australia, "Review of the criminal and civil justice system in Western Australia", 1999,

The content of this article is intended to provide a general guide to the subject matter. Specialist advice should be sought about your specific circumstances.

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