Irrigators have lost a $750 million class action over misplaced water from the Murray-Darling Basin, which they argued would have made their businesses more profitable.
A group of 28,000 irrigators sued the Murray-Darling Basin Authority for negligence, seeking damages over its alleged mishandling of water releases during a prolonged drought.
During a water transfer from the Murray River near Yarrawonga, Victoria, in late 2017, an estimated 40 gigalitres was "lost" to a nearby forest, the NSW Supreme Court heard.
Under other "overbank transfers", a mechanism designed to ensure water returns to the river further downstream, at least 100 gigalitres was lost again to the same forest in late 2018.
Less water would have been lost if the releases occurred in June 2018, but the authority stalled because it was "hoping for rain", the irrigators argued.
They told the court they received 51 per cent of their full water entitlement in 2017-18 and none the following year.
"The evidence makes clear that every megalitre matters for businesses like those carried on by the plaintiffs," NSW Supreme Court Justice Tim Faulkner said in his judgment, handed down on Monday.
"Had they received higher entitlements their businesses would have been more profitable."
However the judge dismissed their claims, finding the authority had a legal obligation to act in the public interest, rather than for private irrigators.
Any duty of care for the irrigators had the potential to "distort" the authority's independent role in overseeing water sharing between the states, the judge ruled.
"The intention that the water of the Murray-Darling Basin be managed in the public interest is an unpromising framework to look for a duty of care being owed to individuals or classes of individuals such as the plaintiffs," Justice Faulkner said.
A duty of care to irrigators would also raise difficulties because the authority acts in the context of conflict, the judge said.
Though the authority had measures to deal with conflicts, disagreements would arise in the "unpredictable, uncertain and dynamic" Murray River system, he said.
"Decisions made by the authority may benefit some interests and prejudice others."
He ordered the irrigator group pay the authority's legal costs of the case, which has been running for many years.
