Much of NSW has been lashed by torrential rain since last week, prompting a spill-over of an already full Warragamba Dam in western Sydney that's likely to last a week.

The spill-over - amid numerous SES evacuation orders for suburbs around the Hawkesbury, Nepean and Colo Rivers - has renewed debate about raising the Warragamba Dam wall to reduce flood risks.

WaterNSW plans to raise the wall by 17 metres to capture additional water and reduce flood damage by an average 75 per cent. The NSW cabinet has not yet agreed to fund the proposal.

The modification would give Hawkesbury Nepean Valley flood plain residents more time to evacuate before floodwaters were released.

But The Colong Foundation spokesman Harry Burkitt said the plans would have made no difference to this week's flood response, and the dam would have spilled over regardless.

The best solution was to ditch government plans to double the flood plain population and instead buy up low-lying Hawkesbury Nepean Valley properties when they come onto the market.

He accused Western Sydney Minister Stuart Ayers - a chief proponent of the dam-raising proposal - of "trying to take full political advantage" of the floods by spruiking the policy this week.

Some 70,000 flood plain residents are in harm's way.

"It's a dangerous plan," Mr Burkitt told AAP on Tuesday.

"Flood plains are for floods and this flood plain has a huge amount of tributaries coming into it - this is not just about the Warragamba River.

"(That) can't be mitigated by any raised dam wall."

Mr Burkitt also said the government should look to reduce the volume of water held in Warragamba Dam, even if it only plays a minor role in flood mitigation.

Deputy Premier John Barilaro indicated he was open to such a proposal, admitting to the Seven Network the raising of the dam wall was "an engineering feat that hasn't been seen before".

Mr Ayres on Tuesday stood by the proposal, saying it would reduce flood peaks and ensure locals could be evacuated safely.

Hawkesbury City Councillor Nathan Zamprogno also supports the project.

"Many people who object to this proposal are doing so from a position of relative safety ... they are high and dry and out of harm's way and it's a bit rich for them to pontificate to us about what we need to make our community safer when we are under water," he told AAP.

Premier Gladys Berejiklian said construction below the one-in-100-year flood line was already prohibited.

Environmental groups and Indigenous locals say parts of the Blue Mountains national park - including up to 1000 hectares of World Heritage area - would be inundated if the dam-raising proposal proceeded.

This could also affect more than 1200 Indigenous sites and artefacts.

Australian Associated Press