The NSW government late on Wednesday (October 17) secured support for a bill in Parliament that will allow the flooding of as much as 4700 hectares of world heritage-listed Blue Mountains land.

The amended legislation was vital to allow the lifting of the dam wall of Sydney's main reservoir at Warragamba by 14 metres to help reduce flood risks in the Hawkesbury-Nepean rivers downstream.

“The government has made clear its intention to do this,” Justin Field, Greens urban water spokesman, said, adding that opponents may have no choice but to take direct action. “The only way to oppose it is to stand in front of it.”

At risk among the 65 kilometres of wild rivers is Kowmung, irreplaceable Aboriginal heritage in the river valleys and endangered species, Mr Field said. While close to Sydney, many of the areas are inaccessible to the public because they fall within Sydney's water catchment.

Stuart Ayres, the Minister for Western Sydney, has previously stated that lifting the wall would temporarily flood only 0.06 per cent of the world heritage area, and help prevent “a catastrophic flood” in the fast-growing part of the city.

A spokesman for Gabrielle Upton, the NSW Environment Minister, said that to ignore the “huge risk” of flooding would be “simply irresponsible”.

“The raising of the Warragamba Dam wall is not going to permanently increase water levels,” he said. “Therefore, any impact on the environment is expected to be minimal and short lived.”

Labor and the Greens, though, have been critical of the lack of scrutiny of the plan. The business case for what could become a $1 billion project is yet to be completed, and the environmental impact statement is not due for release until the middle of next year.

“The community should be disgusted by the process,” Mr Field said. “The government and conservative crossbench members voted for legislation when they have no idea about the full extent of the impacts of this proposal.”

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Harry Burkitt, campaign manager for the Give a Dam group, said there hadn't been such “a blatant disregard” for a world heritage site in Australia since the Franklin Dam proposal almost four decades ago.

“Western Sydney communities don't want to see an additional 134,000 extra people living on western Sydney floodplains, and people don’t want to see a world heritage site destroyed by a dam,” Mr Burkitt said. “This may well be the Franklin campaign of this generation.”

A proposal in 1978 by the Tasmanian government to dam the Gordon River in that state's south-west drew protests at home and abroad, inspired in part by images taken by photographer Peter Dombrovskis.