Wollondilly Council and Blue Mountains Council have joined forces to rally against the state government's plan to raise the Warragamba Dam wall.
The mayors of each council area hosted a community forum on Sunday at the Blue Mountains Theatre and Community Hub in Springwood.
Wollondilly mayor Matthew Deeth said he was happy to work alongside Blue Mountains mayor Mark Greenhill to oppose the proposal and protect people, property and wilderness.
"We are asking the state government to undertake thorough engagement with the local communities and other stakeholders and to consult properly with the indigenous community whose cultural sites will be impacted," he said.
"We are also asking the government to fully examine the environmental impact the raising of the wall would have and to seriously look at other options to address flood mitigation."
Gundungurra woman and Warragamba resident Taylor Clarke, 21, said she was "angry" about the proposal.
"I will continue to be angry, for as long as we have to fight for this, because we shouldn't have to fight," Ms Clarke said. "As we take our next steps forward in this campaign, and indeed as a community, we need to think about the people who are going to walk this land after us. What precedent are we setting?
"When we win this campaign, think of the amazing example we are setting for the next generation, that we are willing to fight to protect our natural heritage.
"And heaven forbid, if we don't (win this campaign), what are we showing the next generation? That we're willing to destroy a National Park for development. To put people in danger. To put people's lives on the line, for development."
Blue Mountains mayor Mark Greenhill said the Warragamba Dam proposal was immoral.
"We strongly urge the NSW Government to cease pursuing its plans to inundate this highly valuable and significant area," he said.