The spill of water over the Warragamba Dam wall on Thursday morning should serve as an urgent warning: much of western Sydney would be devastated by flood if there was another major downpour now.

This caution comes from Stuart Khan, associate professor at the school of civil and environmental engineering at the University of NSW, who has been telling the state government for three years that it should never let Warragamba rise to full – because ample space is needed for flood mitigation.

"I do get worried because I always look back at what happened in July 1998 … [when] about a thousand gigalitres flowed into that reservoir over about a couple of weeks," Associate Professor Khan said.

"It was just pure lucky that … Warragamba was half empty. There was a thousand gigalitres of space to hold that water.