Posted on July 28, 2015 by

Flawed water model – not sufficient for High Court to intervene in long running water dispute

An application for special leave to appeal the Court of Appeal’s decision in Arnold v Minister Administering the Water Management Act 2000 [2015] HCASL 115 (‘Arnold) has been rejected by the High Court. The Arnold decision determined that the NSW Government’s decision to reduce NSW water entitlements, in part based on what was submitted to be a flawed water model, was not manifestly unreasonable and was lawful.


The background to the matter is set out in our previous post.

Current law

Given the High Court’s refusal to grant special leave, the law remains that:

  • the ability of affected persons to successfully challenge decisions on the basis of manifest unreasonableness is not unlimited. In Arnold the Court held that the decision of the Minister to make a water sharing plan was not irrational or manifestly unreasonable as there was material of sufficient probative value to enable him to make the Plan, notwithstanding the flaws in the relevant water model (‘Model’) used by the Minister when making his decision,
  • decisions made on the basis of imperfect science or scientific modelling does not automatically give rise to a finding that those decisions are invalid. In Arnold the Court held that despite the significant flaws of the Model, it had some probative value and it was capable of use, at least as an interim measure, in determining an appropriate water extraction limit,
  • whether misleading advice to a decision maker will invalidate the decision depends on the facts and circumstances of the case. In Arnold the Court held that misleading advice regarding the Model was not material because there was other information before the decision maker that was sufficiently accurate to found a valid decision.

The High Court’s decision can be accessed here.