Plans to raise Warragamba Dam to cut flood risks to downstream communities have been challenged after the leaking of charts show a 14-metre lifting of the wall provides moderate benefit during the biggest events.

The charts, compiled for the environmental impact statement being prepared for Infrastructure NSW, show the anticipated billion-dollar project offers partial reduction for the so-called probable maximum flood, or worst-case scenario.

A similar moderate outcome is also shown for a one-in-500-year flood. Such an event is touted by Infrastructure NSW as similar to the 1867 event, the worst since European settlement to hit the Hawkesbury-Nepean valley.

Warragamba Dam, now sitting at about 50 per cent full. The government says lifting the wall by 14m would reduce downstream flood damage. Credit:Wolter Peeters

The biggest benefit of the 14-metre raising of the wall, standing at 142-metres, comes with moderate flooding in the order of one-in-10 year events, according to a person familiar with study.