A multi-year fight has begun over the Berejiklian government's plan to raise the Warragamba Dam wall amid claims opponents are risking hundreds of thousands of residents to floods. Critics point to evidence of unnecessary haste as reservoir levels sink.

The government last week introduced an amendment bill into the upper house to permit the flooding of more than 3000 hectares of the Blue Mountains World Heritage area should the wall be raised the proposed 14 metres, and Sydney's main reservoir then fill up.

The bill is the start of a "logical sequence" to be followed by an Environmental Impact Statement expected by mid-next year. A business case will also be needed for a project that in 2015 was estimated to cost $670 million, with construction then expected to be finished by 2024.

Warragamba Dam at almost 98 per cent full in June 2016. The government wants to lift the wall by 14 metres to add flood mitigation to the dam's role. Credit:Edwina Pickles

Labor, the Greens and environmental groups have condemned the bill's winding back of protection in the national park that would destroy dozens of Aboriginal sites and 65 kilometres of wild rivers in the region even with temporary inundation.