The Herald can reveal Water Minister Phil Costa will today announce extra offset measures to address social, economic and environmental impacts of the proposed $477 million dam.

The package includes:

? A 1323-hectare national park.

? The release of 2.5 billion litres of water in the Williams estuary each year to improve its flow.

? Modifications to Seaham weir to increase flows to the Hunter River estuary in dry periods.

? $1 million for road and bridge works in Dungog Shire.

? A commitment from Hunter Water to undertake all necessary road maintenance between Dungog and the dam site during the construction of Tillegra.

? The relocation of the Bendolba Rural Fire Station to a site requested by the Rural Fire Service.

The major project modifications are outlined in Hunter Water's Submissions Report, to be lodged with the NSW Department of Planning today.

Hunter Water managing director Kevin Young said the report would "put to bed" the key issues individuals and community groups had raised in their 2669 submissions to the project.

"All of the modelling shows there won't be any significant impact on the wetlands, or the estuary downstream, or the prawn and fishing industry," he said.

"Hunter Water did not use climate change calculations in its justification of the project, it used historical data, and the Government agencies that questioned the data now agree with it."

However, Mr Young said Tillegra was not impact-free.

He said the platypus population in the river section set to be dammed would be affected and the five property owners who are yet to sell to Hunter Water would be affected.

"They will be impacted by this proposal going ahead but we believe if you weigh everything up you'll see Tillegra is the most best value for the community in terms of drought security and future generations," Mr Young said.

"It's safe and it's sustainable, therefore it's a project that deserves to go ahead."

The planned national park comes on top of a promised 1500 hectares of vegetation in a biodiversity corridor.

The submission report will be available on Hunter Water's website from today.