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.

Gundunguarra traditional owner Kazan Brown on Friday said hundreds of cultural sites would be flooded by any additional raising of the wall.

"We were horrified when they announced their plans to raise the dam by 14 metres - now it's 17," Ms Brown said in a statement.

The government wants to raise the dam wall to allow additional floodwater to be captured and temporarily held back, giving residents on the Hawkesbury-Nepean Valley flood plain 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.

Western Sydney Minister Stuart Ayres told a state parliamentary committee in September 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."

The Greens on Friday accused Mr Ayres and Infrastructure NSW of misleading parliament.

"Despite repeated questioning, the minister continued to give an answer that now seems to be covering for plans to prepare for a higher wall," Greens MP Justin Field said in a statement.

NSW Opposition leader Michael Daley on Friday argued the wall raising was a "Trojan horse" for more development on the flood plain.

"We are hearing first hand from the media about plans this government has without consulting residents (and) without being transparent," the Labor leader told reporters.

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 says.

"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 partly to increase community resilience.

AAP on Friday asked Infrastructure NSW if the structural raising to 17m would be made public and assessed in terms of its potential environmental impact.

The agency replied: "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."

Australian Associated Press