Liberal MP Kevin Connolly argued the nearly $2 billion project pushed by Premier Dominic Perrottet last week "is the best way to protect people, homes and businesses of western Sydney".
"It is the option that produces the greatest reduction in the height of floods in the (Hawkesbury-Nepean) valley," the MP for Riverstone in western Sydney said.
"It is an option in an increasingly variable climate which optimises both water storage and flood mitigation."
He said the costly project can be completed in four years.
The public interest debate, sparked by a motion from Mr Connolly, comes on the eve of another downpour for a state that has endured four floods in the past 18 months.
Sydney is in the midst of its wettest year on record as widespread rain puts many parts of western NSW on flood alert.
The previous annual record of 2194mm, which had stood since 1950, was broken last week.
Experts such as Floodplain Management Australia President Ian Dinham praised the government's announcement of raising the dam as a "critical state significant infrastructure".
"There are important environmental and cultural issues to consider in raising the dam and these must be effectively managed, but at the end of the day this is about saving lives," Mr Dinham said.
Mr Perrottet said the project would put "people before plants".
But NSW Labor opposed Tuesday motion, saying raising the dam's wall was a piecemeal solution to flood mitigation.
Labor's Greg Warren said the "government's own flood inquiry did not recommend this project".
"They said the best thing to protect lives is to stop building on the flood plain," the Campbelltown MP said.
Blue Mountains Labor MP Trish Doyle also questioned the feasibility of the structural work, calling it "a dumb idea" and "reckless".
"Even if the wall is completed, the threat is not diminished because 45 per cent of flood waters come from the Colo, Grose and the Nepean Rivers," she said.
Former Western Sydney minister Stuart Ayres argued the project would save 5000 homes that are in the path of more floods.
"We're not going to spend billions and billions and billions of dollars relocating those people from low cost housing to housing they can't afford," the Liberal MP for Penrith said.
"We can't remove flood risk completely on a flood plain. We're not God.
"We can't stop it from raining, but what we can do is learn from what science and engineering has told us to reduce that risk."
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