Numerous tree roots and a wrong plumbing connection have been blamed for homes at Caringbah experiencing six sewage overflows in the last year.
The Leader revealed the plight of the residents in Carabella Road, who said Sydney Water had failed to solve the problem despite numerous requests.
During the latest incident 10 days ago, the overflow extended further than previously, into the yards of homes in Taronga Parade.
Sydney Water, which promised "a full and through inspection", has written to residents about the findings.
"We apologise for the repeated wastewater overflow incidents during heavy rain in your area," the letter said.
"We engaged specialist contractors to clean, disinfect and deodorise the affected areas, and we're now reviewing the overall performance of the wastewater system in the area to help prevent this happening again.
"We inspected the inside of the wastewater pipes and found that numerous tree roots had entered the pipes seeking moisture.
"Some of these tree roots had entered through our mains, while others entered through household plumbing and grown into our pipes.
"We've now cleared these blockages with specialised high-pressure water cleaning equipment.
"We also found that pipes from one house had been incorrectly plumbed into our wastewater main, partly blocking it.
"We fixed this using a specialised robotic grinding machine."
Resident Betty Gregory thanked the Leader and the Labor candidate for Cronulla Teressa Farhart for getting action.
"You guys have done an amazing job of bringing this problem to light," she said in an email.
"All the residents affected are so grateful.
"[Sydney Water] was very busy over the following days, putting cameras down the sewer system and trying to find out where the problem lied.
"They checked each line in each property."
