The NSW government could raise Warragamba Dam wall higher than the initial 14 metres proposed, which experts say would have a greater environmental impact.
The NSW government is considering raising parts of the Warragamba Dam wall significantly higher than what they've admitted to publicly.
Government documents seen by AAP show the wall will be designed so it can hold back an additional 17 metres of water even though the coalition initially plans to raise the flood level by just 14m.
Despite this, the government isn't assessing the environmental impact of raising the wall an additional three metres.
The documents show the Berejiklian government plans to raise each end of the wall - the dam abutments - by 17m with the spillway initially set at 14m.
The design would allow the spillway to be modified in the future if climate change leads to increased flooding.
Essentially the wall will be "structurally" raised 17m but "operational" at 14m.
Raising the dam wall would allow additional floodwater to be captured and temporarily held back, giving residents on the Hawkesbury-Nepean Valley floodplain extra time to evacuate before a controlled release.
But AAP understands the 14m raising wouldn't actually provide authorities enough time to evacuate residents.
Premier Gladys Berejiklian on Friday was quizzed about the proposed height of the dam wall.
"I ask you to rely on what's in the public domain - that's what we've announced (and) that's what's happening," she told reporters.
"When it comes to saving lives and saving properties in western Sydney this has to happen."
Western Sydney Minister Stuart Ayres told a state parliamentary committee in September 2018 the business case was looking only at a 14m raising.
"We are very conscious of the environmental impacts that would occur upstream were we to raise it by more than that," he told the hearing.
"Fourteen metres is what we are preparing a business case and an environmental impact statement for."
If the dam wall was raised an extra three metres it could cause significantly more damage upstream in the heritage-listed Blue Mountains, Australian National University water expert Jamie Pittock said.
"It would mean tens of kilometres of more inundation," Professor Pittock told AAP.
The NSW government is using its climate change fund to pay for the design and environmental assessment of the Warragamba project.
The fund was set up to address the impacts of climate change and to increase community resilience.
The government argues raising the dam wall will make downstream communities more resilient, but environmental campaigners argue the project is an unacceptable use of those funds.
Nature Conservation Council chief executive Kate Smolski insists raising the wall is "environmentally harmful".
Independent think-tank the Grattan Institute, however, argues the project isn't completely unrelated to climate change because it aims to mitigate extreme weather events.
"It is stretching things a little bit ... but there is logic to it," energy program director Tony Wood told AAP.
A UNESCO advisory body claims the 14m proposal could flood up to 1000 hectares of world heritage area and 3700 hectares of surrounding national park, and could put the area on a list of heritage in "danger".
The NSW parliament in October 2018 passed legislation allowing flooding of the Blue Mountains.