Two hydrology reports commissioned by the NSW Department of Planning have criticised the water authority’s assessment of the dam’s environmental impact.

The reports reject Hunter Water’s conclusion that the dam would have ‘‘no measurable effect’’ on the Hunter estuary and Williams River, and back a June report saying it would seriously damage one of Australia’s most important bird-breeding areas at Kooragang Wetlands.

The reports from August make three out of three negative reports commissioned by the Department of Planning in the lead-up to a NSW government decision on Tillegra, after a socio-economic report found the dam was not the best solution for the region’s future water needs.

Independent consultants Bewsher, and the University of NSW’s Water Research Laboratory, rejected the validity, accuracy and interpretation of Hunter Water’s modelling of changes to water flows on the Williams River, the Hunter Estuary and wetlands.

The report found existing water infrastructure on the Williams River and anticipated impacts from the dam would reduce water flows to the estuary by 43per cent.

‘‘NSW Planning Minister Tony Kelly and Federal Environment Minister Tony Burke could not now approve the $477-million project unless prepared to take a massive gamble on the future survival of the Hunter wetlands and the Williams River,’’ Greens MP John Kaye said.

A Hunter Water spokesperson said its model provided an excellent estimate of daily and monthly flows.

‘‘The Bewsher report notes that Hunter Water’s hydrodynamic modelling is appropriate for determining the potential impacts of Tillegra Dam and, in fact, overestimates any potential risks to the estuary,’’ they said.

‘‘These reports, when complete, will be considered by NSW Planning as part of its merit review.’’