Of all the debates swirling over Sydney's burgeoning population and where to house an extra 80,000-plus residents each year, few feature the height of a dam wall on the city's western edge.

But Bob Debus aims to change that, hosting a public debate in Springwood on Monday to challenge the wisdom of lifting Warragamba's dam wall by 14 metres.

The real motivation for the $700 million project is to open the way for more development in the flood-prone Hawkesbury-Nepean valley.

Lake Burragorang, behind Warragamba Dam, Sydney's main reservoir. Credit:Jonathan Ng

“Its principal purpose is to allow more development in an area where it should never be allowed,” Mr Debus, who served as NSW environment minister for eight years, told Fairfax Media.