Flooding in western Sydney shows a plan to raise Warragamba Dam isn't a cure-all solution, experts say, given no water spilled over the dam wall this weekend but thousands were still told to evacuate.
Heavy rainfall over the weekend caused parts of the Hawkesbury-Nepean Valley to become inundated with roads closed and evacuation warnings issued.
The NSW government controversially wants to raise the dam wall by at least 14 metres to potentially hold back additional water in the Blue Mountains.
But during the weekend areas in Windsor, Sackville and Wallacia were flooded when the Hawkesbury River peaked despite Warragamba Dam being only about 50 per cent full.
"The rain caused floods down all the tributary rivers downstream of Warragamba, while not a drop has spilled over the dam," Australian National University flooding expert Professor Jamie Pittock said on Tuesday.
The "real-life experiment" showed the coalition's push to raise the dam wall was flawed, Prof Pittock told AAP.
"The government's flood strategy is relying solely on raising Warragamba Dam wall which would expose residents to flood risk," he said.
The dam mostly captures water from the catchment while the Hawkesbury River and its tributaries - such as the Nepean and the Grose rivers - fill up at choking points and eventually spill over during heavy rain events.
Prof Pittock says because even moderate floods - including those on the weekend - cause so much disruption the government should build better roads and evacuation routes.
He suggests restricting development on the floodplain to keep people safe.
"There have been many damaging floods that did not involve floodwaters coming past Warragamba Dam," Prof Pittock said.
"They shouldn't proceed with raising the dam wall. The key is to stop building suburbs in flood-prone areas."
Former NSW SES deputy director-general Dr Chas Keys says raising the wall will have some mitigation benefits but it's not the "be all and end all".
During heavy rainfall, water rises at "heroic" levels at choking points on the tributaries feeding the Hawkesbury River.
"Whatever they do at Warragamba will be utterly irrelevant if there's flooding at the Nepean and Grose rivers," Dr Keys told AAP.
The Berejiklian government is "mad" to be doubling the population of the Hawkesbury-Nepean Valley when the threat of extreme floods will remain even if the wall is raised, he said on Tuesday.
The Bureau of Meteorology on the weekend tweeted a video about the "bathtub effect" which makes floods in the Hawkesbury-Nepean Valley more dangerous than elsewhere.
The video explains choke points where the tributaries of the Hawkesbury River narrow causing floodwaters to back up in the valley.
NSW Minister for Western Sydney Stuart Ayres insists the flooding would be more severe without a raised dam wall.
"The speed at which an east coast low can fill Warragamba Dam is breathtaking. The dam has jumped almost 30 per cent in a little over four days," he told AAP in a statement on Tuesday.
"The simple fact is; had the dam been above 80 per cent with no flood mitigation wall, the Hawkesbury-Nepean would be in a lot of trouble."
He said any suggestion raising the wall would allow more development on flood-prone land is "as dangerous as it is false".
Australian Associated Press