A senior SES official has warned that Parramatta's CBD could be overwhelmed by a flash flood in as little as nine minutes, giving residents little time to evacuate.

George Jeoffreys, senior manager for risk reduction and avoidance at the NSW State Emergency Service, cited the difficulty of predicting flash floods.

Flooding on James Ruse Drive at Parramatta on February 8. Credit:Steven Issa

"The risk is the inability to get people out. Our warning times can be short," he warned.

"If a severe super cell develops over the [Parramatta] area, we would have nine minutes from when the rain starts falling before floodwaters hit the CBD. We would not have time to evacuate. It becomes a major rescue operation."