Dozens of homes at Woronora were inundated in a "crazy" 30-minute period of flooding not seen in the suburb for at least 50 years.
At 4.30pm on Monday, the river was just spilling over the banks as a result of the rain bomb which had hit Sydney and the Woronora Dam reaching capacity.
However, with low tide due at 5.18pm, residents had little concern.
The torrent of water, which gushed down the river as residents were getting ready for dinner, caught everyone by surprise.
By 5.30pm, the water had spread across grassed areas and into homes, saturating furniture, flooring, bedding, appliances and other equipment, and covering everything with mud.
The situation has caused many residents to speculate there was a deliberate release from Woronora Dam timed to hit the area at low tide when there would be less impact.
However, the dams authority denied this was the case.
"WaterNSW made no operational releases from Woronora Dam during the recent flood event," a spokesman said.
"The dam storage exceeded capacity and spilled into the downstream river on Sunday as a result of significant inflows generated by rainfall.
"Ongoing intense rainfall generated an increase in the rate of uncontrolled spill from the dam Monday afternoon."
Flooding also occurred at Woronora on Saturday, with several cars going under and some homes impacted.
The challenging situation has produced great community spirit, with residents quickly coming to the aid of their neighbours in cleaning up the mess.
Frank Powell, 84, who lives on the edge of Forbes Creek and had half a metre of water through his lounge room, said, "I would have been lost without my neighbours".
"They were in here with mops, buckets and hoses," he said. "Without them, I would still be sitting in mud."
Travis Bates, who lives in Prices Circuit, said "heaps of people" came to his assistance, which included towing his last edition VF Series Holden Commodore from his garage on Saturday night while he was away.
While the Commodore, valued at $80,000, appears to be a write-off due to water damage, neighbours saved his work truck.
Mr Bates, a builder, said fortunately his wife and their two young daughters were in Melbourne.
"The kids' bedroom is a total mess," he said.
Mr Bates said the flooding occurred so quickly.
"It was crazy," he said. "Debris was flowing down the river at 15-20 knots."
Margaret and Peter Adams, who live next door, said they had been in their home for 26 years and "never seen anything like this".
"Water came from both directions - the river and the road," Mrs Adams said.
"We have been flooded twice before, but both times it was from the road [stormwater pipes overflowing], never from the river, not even close."
However, the couple consider themselves fortunate.
"Compared to what the people in the Windsor area are going through, we have nothing to complain about," Mr Adams said.
Jane Costello, who lives in Liffey Place on the eastern side of the river, said it was the first time her house had been flooded in her 52 years at Woronora.
Mrs Costello said the floodwater came from the street, rather than the river, and entered the downstairs area containing bedrooms and family area.
James Chalker, in Thorpe Road, said, "We had just cleaned up after Saturday, and it came again. It happened in the space of about 15 minutes".
The Boatshed Woronora's popular cafe is a disaster zone.
Manager Yasmine Fewtrell said the water level reached waist high.
"We lost everything in the kitchen, but our trusty coffee machine made it through," she said.
There is a thick coating of mud throughout, but the team's spirits are high.
"We are aiming to reopen at the end of this month," Ms Fewtrell said.