The NSW Land and Environment Court has found two well-known irrigators illegally took water from the Barwon River in north-west NSW in 2016.
Peter and Jane Harris will be sentenced on a date to be fixed by the court after which they will have 28 days to appeal the decision.
The court found Mr and Mrs Harris illegally extracted water for irrigation from the Barwon River during June 2016.
The couple's actions were contrary to a condition of their joint water use and water supply approvals under section 91(G) of the Water Management Act 2000 (NSW).
In 2018 WaterNSW brought legal proceedings against the pair for breaching a condition of their approvals associated with the water licence for Beemery Farm, a property on the Barwon River at Brewarrina.
The proceedings were heard by the court in February 2019 with the findings handed down last week.
In a statement, WaterNSW chief executive officer (CEO) David Harris said in the 18-month period from July 2016 to April 2018, when the state-owned corporation was still responsible for compliance, staff conducted several detailed investigations "including two matters" which resulted in it commencing prosecutions in the Land and Environment Court.
The CEO said the convictions demonstrated the commitment and diligence WaterNSW brought to the role of ensuring rules for extracting water were enforced before the compliance role was transferred to the Natural Resource Access Regulator.
He said the result of the court case should also serve to "reassure the broader community that water authorities are serious about managing water resources strictly according to the rules".