Hot Tub Done Right: A Rare Division 1 “How-To” Ruling on Concurrent Expert Evidence in Parenting Trials

In Ashdown & Markin (No 2) [2026] FedCFamC1F 164 (Wilson J, 16 March 2026), the Court delivered a highly practical ruling explaining when and why four medical experts should give evidence concurrently (“in the hot tub”) in a parenting trial involving serious family violence allegations and disputed psychological risk evidence. Although the final orders were made by consent, the reasons function as an unusually clear procedural guide for practitioners: the Court grounded the “hot tub” decision in (i) expert duties to the Court, (ii) the Evidence Act admissibility principles for expert opinion, (iii) the Family Law Act’s case-management duties and relaxed evidence regime in parenting matters, and (iv) the overarching statutory obligation to conduct parenting litigation quickly, inexpensively and efficiently.

🧩 Facts and Issues

Facts: This parenting proceeding commenced in 2022 (Division 2) and was later transferred to Division 1. The trial was vacated twice in 2025 (including for updated expert assessments and because earlier time estimates were unrealistic). The matter ultimately proceeded in March 2026 as a 10-day trial. There were extensive allegations of family violence and competing views about the father’s psychological state and future risk. Four medical experts (three psychologists and one psychiatrist) produced reports that diverged on key matters such as diagnosis, risk, insight and change.

Because the lay evidence had already overrun its planned duration, the Court explored whether the expert phase—estimated to take at least two full days if done sequentially—should instead be taken concurrently. After a recusal application was dismissed, the judge raised the question and the parties took instructions. Ultimately, orders were made by consent for concurrent evidence.

Issues:

1. In a Division 1 parenting trial, when is it appropriate to take expert evidence concurrently rather than sequentially?
2. What principles and safeguards ensure procedural fairness when experts are “hot tubbed”?
3. What procedure should be used (conclave, list of issues, questioning sequence) so the Court receives the “totality” of expert evidence efficiently and accurately?

⚖️ Applicable Law – Legislation, Regulations, Rules

Evidence Act 1995 (Cth)

• s 79 — expert opinion rule: expert evidence must come from a person with specialised knowledge and be based on that knowledge; the Court referenced High Court and NSWCA authorities on the proper ambit and limits of expert opinion (including the “Makita” principles).

Family Law Act 1975 (Cth)

• s 95(1)(d) — overarching purpose in parenting litigation: conducted “as quickly, inexpensively and efficiently as possible”, with a duty on parties and representatives to facilitate that purpose.
• s 102NE — the trial judge must actively direct, control and manage the conduct of the parenting proceeding.
• s 102NL — in Division 1 parenting cases, the strict rules of evidence are relaxed, supporting flexible procedures (including how expert evidence is received), so long as fairness is preserved.

📌 Precedents Relied On

Expert evidence principles (admissibility / methodology)

• Dasreef Pty Ltd v Hawchar (2011) 243 CLR 588
• Honeysett v R (2014) 253 CLR 122
• Lang v R (2023) 278 CLR 323
• Makita (Australia) Pty Ltd v Sprowles (2001) 52 NSWLR 705

Concurrent evidence / practice

• Willans & Enmore (No 2) [2021] FamCA 340 (noting limited jurisprudence in this Court)
• Two Federal Court judicial papers used as practical guidance:
• Middleton J, Concurrent expert evidence: still flavour of the month? (2018)
• Rares J, Using the “Hot Tub” (2010)

Context

• Ashdown & Markin [2026] FedCFamC1F 156 (recusal ruling earlier in the same trial)

🧠 IRAC Analysis

Issue

Whether (and how) the Court should take the evidence of four mental-health experts concurrently in a parenting trial where:

• experts disagree materially on diagnosis and risk, and
• sequential cross-examination risked inefficiency and the Court receiving an incomplete or distorted picture.

Rule

1. Expert evidence must assist the Court, and experts owe duties to the Court rather than to the parties; they should not act as advocates or “hired guns”. The Court must ensure each expert is properly qualified and their opinion fits within s 79 Evidence Act and the accepted methodology principles (Dasreef; Honeysett; Lang; Makita).
2. In Division 1 parenting proceedings, the judge has an affirmative duty to actively manage the hearing (s 102NE) and the rules of evidence are relaxed (s 102NL), but procedural fairness remains essential — counsel must have a fair opportunity to question all experts.
3. The Court should conduct parenting litigation “as quickly, inexpensively and efficiently as possible” (s 95(1)(d)). If the conventional sequence risks wasted time or leaves the Court “at risk of being misinformed” because contradictions emerge only after an expert has left the witness box, concurrent evidence can be the better procedure.

Application

• Why sequential evidence posed a real forensic risk: Wilson J identified a practical problem with conventional expert evidence: once Expert 1 is cross-examined and excused, Expert 2 may later give contrary evidence and Expert 1 may wish to refute it with probative material not earlier thought necessary. Without leave to recall, the Court may be left with an incomplete or misleading understanding. His Honour described that risk as “wholly undesirable” and something that should be avoided where possible.
• Why concurrent evidence fit this case: The proceeding involved competing mental health opinions about (i) diagnosis, (ii) the risk of aggression and fear, (iii) treatment success, and (iv) risk/protective factors — precisely the kind of topic where side-by-side testing and expert-to-expert questioning can expose assumptions and sharpen agreement/disagreement efficiently.
• Fairness built into the hot tub: The Court emphasised that “one size does not fit all” but anchored the process in procedural fairness — every party had a fair opportunity to question each expert. The judge set out a structured procedure (swear/adopt reports; opening précis; expert-to-expert questions; then counsel questions; repeated for each expert; final “any point” opportunity), and invited counsel to object or propose changes; none did.
• Conclave + list of issues (the key discipline): The experts were required to confer (a “conclave”) and prepare a document identifying:

1. distilled issues raised by lay evidence,
2. matters agreed, and
3. matters disagreed.
4. The ICL’s counsel took carriage of that list. The list then became the organising spine of the hot tub.

• Outcome: dramatic efficiency without sacrificing content: The entire expert evidence phase concluded within three hours on the allocated day — demonstrating why the Court viewed hot tubbing as consistent with the statutory imperative of efficient parenting trials.

Conclusion

The Court ordered (by consent) that four experts give evidence concurrently and provided reasons that operate as a practical “benchbook” on how to do it properly in Division 1 parenting litigation: conclave, list of issues, judge-controlled structure, expert-to-expert engagement, and full counsel opportunity, producing a faster and more reliable synthesis of competing expert opinions.

🧠 Take-Home Lesson

This decision is valuable because it explains, in plain and operational terms, why concurrent expert evidence can be superior in parenting trials where experts fundamentally disagree on risk: it reduces the chance the Court is left with a fragmented expert picture (because experts are not easily recalled), and it advances the Family Law Act’s direction that parenting matters be run quickly, inexpensively and efficiently—while still preserving procedural fairness through structured questioning and equal testing of each expert.