In alarming research, experts have warned about a massive flood risk in Parramatta.
Nine minutes is all it would take for the region to be swamped by a catastrophic flash-flooding event, the Sydney Morning Herald reports.
NSW SES Senior Risk Reduction Manager George Jeoffreys says warning times "can be short", which poses the largest risk.
"If a severe super-cell develops over the area we would have nine minutes from when the rain starts falling before floodwaters hit the CBD," he said.
"We wouldn't have time to evacuate.
"It becomes a major rescue operation.”
Earlier this month, SES workers had to rescue a number of motorists who got trapped when heavy rain saw the drainage system put under pressure.