NSW Labor will oppose plans to raise the wall of Warragamba Dam, saying the government is pandering to developers at the cost of the environment.
The coalition tabled an amendment to the National Parks and Wildlife Act on Wednesday, paving the way for the Blue Mountains dam to be raised by 14 metres, which could temporarily flood parts of the world-famous national park.
The government says this will reduce the overall risk of flooding downstream and more time for evacuation will be needed in the case of major weather events.
But in the event of the dam reaching 100 per cent capacity, higher water levels upstream would see new areas of bushland inundated for up to two weeks.
A Infrastructure NSW spokesman said while the proposal would have "significant community benefits," they added it was important the decision makers understood the potential envronmental and social impacts of the dam being raised.
These assessments are being done properly, with detailed investigations and public consultation - and that takes time," the spokeswoman said in a statement to AAP.
Labor claims the government's real motivation is to open up flood-prone land in northwestern Sydney for development.
Spokeswoman Penny Sharpe says the plan is an "appalling attack" on the Blue Mountains National Park and would allow the flooding of up to 50 square kilometres of the World Heritage-listed area.
"The proposal is the equivalent of dredging the corals of the Great Barrier Reef," she said in a statement on Thursday.
"These laws jumped the gun on the government's own environmental process."
Labor estimates the plan will come with a $670 million price tag.
Water NSW says raising the dam wall would help reduce - but not completely eliminate - flooding in a major weather event.
The NSW Greens on Thursday argued "wild rivers and dozens of important Aboriginal sites could be destroyed forever" if the bill was passed and the dam wall raised.
"To introduce legislation that will allow the destruction of Sydney's iconic wild rivers before the environmental impact assessment and community consultation for the project is even completed shows this government has no respect for the community or nature," urban water spokesman Justin Field said in a statement.
"We need to invest in water efficiency, recycling and re-use, not expensive, environmentally destructive dams.
