Background
The Ramsay family are landowners near Warren, NSW whose properties — “Banchory” and “Flintrock” — sit on a floodplain regulated under the Water Sharing Plan for the Macquarie and Cudgegong Regulated Rivers Water Source 2016. Since 2014 they had pursued replacement floodplain harvesting access licences under s 57A of the Water Management Act 2000 (NSW), which entitle holders to a defined share of available water expressed in unit shares (one unit equalling one megalitre). After years of correspondence and modelling by the Department, the Minister in February 2023 issued licences with share components of 3,926 units for Banchory and 2,522 units for Flintrock. The Ramsays contended the correct figures exceeded 8,000 units for each property and challenged the determinations on grounds of jurisdictional error in proceedings commenced in this Court in September 2023, running concurrently.
A hearing before Davies J was scheduled for late October 2024 but was vacated by consent when the parties reached an in-principle agreement to set aside the determinations. That agreement ultimately fell through. The Minister subsequently made an open offer in April 2026 to concede jurisdictional error — acknowledging a “Boolean error” in Python code used to apply the rainfall run-off harvesting exemption within the plan limit compliance scenario model — set aside the determinations, pay costs, and redetermine the share components, an offer the Ramsays rejected as providing insufficient relief. By the time of this hearing the proceedings had been relisted for substantive determination, with the Minister’s conceded coding error and the Ramsays’ broader challenges to the eligible works model still unresolved.
By notice of motion dated 4 June 2026, the Minister sought an advance ruling under s 192A of the Evidence Act 1995 (NSW) on the admissibility of four joint hydrology reports prepared by the parties’ respective experts — Dr Daniel Martens (for the Ramsays) and Mr Daren Barma (for the Minister) — comprising original joint reports for each property dated September 2024 and supplementary joint reports for each property dated 23 October 2024. The Minister argued the reports were inadmissible and that further lay evidence from the Department’s modellers and fresh expert evidence were necessary to resolve the hydrological issues at the heart of the case.
The Court’s Holding
Griffiths AJ held that it was appropriate to give advance rulings under s 192A in all the circumstances. The admissibility question would critically inform how the proceedings ran from that point; the issue would inevitably need to be decided; and resolving it now — rather than at the start of trial — would allow deficiencies to be remedied efficiently. The judge rejected the Ramsays’ submission that there were no obvious evidential problems with the reports.
On admissibility, the court ruled all four joint hydrology reports inadmissible. The core defect in each report was the failure of the experts to adequately expose their path of reasoning, to identify the assumptions upon which their opinions rested, and to show how those assumptions and the underlying facts led to the conclusions expressed. This is a requirement for expert opinion evidence to be admissible under s 79 of the Evidence Act 1995 (NSW): an expert’s opinion is only admissible if the basis for the opinion — the facts, data, and reasoning — is sufficiently disclosed so that the court can evaluate it. The supplementary reports, which had been produced specifically to address identified deficiencies in the originals, did not cure those deficiencies.
Having ruled the reports inadmissible, the court granted leave to both sides to file fresh evidence on an agreed timetable: the Minister to file lay affidavits and an expert report from Mr Barma by September 2026; the Ramsays to file further lay and expert evidence from Dr Martens by July 2026; and the two experts to confer and produce a new joint expert report by October 2026. The costs of the motion were ordered to be costs in the cause.
Key Takeaways
- A joint expert report that does not adequately identify the assumptions relied upon and does not expose the chain of reasoning linking those assumptions and the underlying data to the experts’ conclusions is inadmissible under s 79 of the Evidence Act 1995 (NSW), even where the experts agree on outcomes.
- An advance ruling under s 192A is appropriate where admissibility is genuinely in doubt, the question is suitable for determination on the existing material, and an early ruling will improve trial efficiency — in particular where it allows a party to remedy deficient evidence before the hearing rather than facing exclusion at trial.
- Conceding jurisdictional error in one aspect of a multi-faceted administrative decision does not necessarily resolve the proceedings: where the applicants challenge other aspects of the decision on broader grounds, they may rationally decline an offer that would merely set aside and redetermine the impugned decision on narrower grounds.
- The complex interplay of hydrological modelling requirements under the Water Management (General) Regulations 2018 (NSW) — particularly the eligible works model under cl 23I — remains intensely contested, with the recent decisions in Budvalt [2025] NSWSC 609 and Emu Rider [2025] NSWLEC 64 providing only partial guidance.
Why It Matters
This ruling is a practical reminder to practitioners preparing joint expert reports in technically complex litigation that agreed conclusions do not substitute for explained reasoning. Courts applying the Evidence Act require that the inferential pathway from data to opinion be transparent and auditable; where it is not, inadmissibility follows regardless of the eminence of the experts or the consensus between them. The case also illustrates the procedural value of the s 192A advance ruling mechanism in long-running, heavily expert-dependent litigation: catching and curing evidential deficiencies before trial avoids the disruption and wasted costs of exclusion at the hearing itself.
More broadly, the proceedings form part of a growing body of NSW litigation over the validity of floodplain harvesting access licence determinations issued under the Water Management Act 2000. The acknowledgment by the Minister of a Boolean coding error in the Python code underpinning the plan limit compliance scenario model — and the Ramsays’ rejection of the consequent remedial offer — signals that disputes about the accuracy and lawfulness of the hydrological modelling process remain live and potentially far-reaching for landowners across the Macquarie Valley regulated river system.