Much of NSW has been lashed by torrential rain since last week, with 20 evacuation orders in place from the mid north coast to the Illawarra and western Sydney.

Those issued for the recently developed Hawkesbury-Nepean Valley, northwest of Sydney, could have been avoided, Australian National University Professor Jamie Pittock says.

"The Hawkesbury-Nepean Valley is very dangerous, very flood-prone and that's because there are some geographical choke points that cause large floods to back up over some of the inhabited areas," he told ABC TV on Monday.

"There is around 70,000 residents on the flood plain that are in harm's way and the NSW government has plans to move another 130,000 people into this area by 2050."

A government plan to raise the Warragamba Dam wall to offset the flood risk is "silly", he said, and will only delay the inevitable.

WaterNSW plans to raise the wall 17 metres to allow additional floodwater to be captured and temporarily held back, pending federal approval.

"It only encourages more development in harm's way," Prof Pittock said.

"The best approach, the sort of approach that is being taken in countries around the world, is to prevent development on the flood plain, to leave the flood plains as places that can flood safely."

"Use that land for agriculture and recreation and nature conservation but don't put more houses there."

A reported war of words between NSW government ministers over whether or not more water from the dam should have been released before the rain is missing the point, he says.

"Letting a little bit of water out of the dam beforehand was an obvious step to take and it would have made some difference."

"(But) no matter what happens at Warragamba Dam there is a big flood risk from places like South Creek and the Grose River."

With climate change driving extreme weather events that make major flooding more common, a plan phased over several decades to relocate those living on the flood plain is needed, Prof Pittock says.

Australian Associated Press