By capturing more of the water in Lake Burragorang behind the dam, residents in the increasingly populated Hawkesbury-Nepean floodplain would have more time to evacuate, one person familiar with the plans said.
The environmental impact assessment has only taken into account the 14-metre wall raising, the person said.
Premier Gladys Berejiklian told reporters on Friday that “what we’ve publicly said is what’s going to happen”, declining to comment of the 17-metre plan.
A higher dam wall “will make sure in the future we also have capacity for extra water storage”, she said.
Harry Burkitt, a campaigner with the Give a Dam group opposed to the wall-raising, said Jim Betts, the chief executive of Infrastructure NSW, “has serious questions to answer”.
“If this report is true, Mr Betts and his department have misled the NSW Parliament and the public about the height of the proposed dam raising,” he said, adding the agency should “come clean about how many thousands of hectares” of the national park will be inundated.
An Infrastructure NSW spokesman said the wall proposal “is being prepared to raise the dam wall by around 14 metres to create ‘airspace’ to capture and temporarily hold back floodwaters”.
“Under NSW legislation, the NSW Secretary of the Department of Planning and Environment has required the project to be ‘designed, constructed and operated to be resilient to the future impacts of climate change,'” he said.
Any modification in the future “would require new environmental, planning and business case approvals, as well as government approval to proceed”, he said.
Justin Field, the Greens water spokesman, said Mr Betts and Stuart Ayres, the Minister for Western Sydney, may both have misled Parliament over dam wall plans.
“During budget estimates hearings I asked the Minister to guarantee the people of Sydney, and Western Sydney in particular, that the dam wall will not be raised by more than 14 metres. He responded to say ‘We are doing our business case on 14 metres,'” he said.
The Herald approached Minister Ayres for comment.
Kazan Brown, a Gundunguarra Traditional Owner, said the plans were “unbelievable”.
“We were horrified when they announced their plans to raise the dam by 14 metres – now it’s 17 metres,” Mr Brown said. “Hundreds of our cultural sites will be flooded by this additional raising.”
Peter Hannam writes on environment issues for The Sydney Morning Herald and The Age.