Sydney Water has launched "a full and thorough investigation" of repeated instances of raw sewage running over lawns, driveways and under houses in an area of Caringbah.
Residents in Carabella Road said sewage overflows had occurred from pipes between their properties on six occasions in the last year, including three since Sunday.
In the latest incident on Wednesday, homes in Taronga Parade were also impacted.
"Sydney Water is conducting a full and thorough investigation to establish the cause of repeated wastewater overflow incidents during heavy rain affecting homes in Carabella Road and Taronga Parade at Caringbah, with an aim to prevent future occurrences," a spokesman said.
"We will urgently conduct a CCTV inspection of the wastewater pipe on Thursday to identify any blockages or faults as well as undertaking a review of system performance.
"Sydney Water crews were on site today (Wednesday) conducting an external clean up.
"A customer advocate has been on site liaising with the affected customers and also arranged for contract cleaners to conduct full wastewater removal, disinfection and deodorisation.
"Sydney Water takes its responsibility to protect public health and the environment seriously."
Raw sewage running over your lawn, down the driveway and under your house... It's unthinkable.
But, that's what has happened six times in the last year, including three times in the last four days, to residents in four adjoining houses in Carabella Road, Caringbah.
This morning, an overflow extended further than previously, into the yards of homes in Taronga Parade.
The sewage overflows occur in very heavy rain and, when the water subsides, yards and driveways are left covered with toilet paper and other disgusting items.
One of the residents is a woman, 88.
"Her home becomes surrounded by a moat of effluent", said neighbour Dale Badger, who is also affected.
"The overflows are from sewage breather pipes that run between the houses.
"It's happened five times since February 11, 2018, including Sunday and Tuesday this week.
"Each time we call Sydney Water, but they have been unable to fix the problem and don't have an answer as to why it is happening."
Betty Gregory, another affected resident, said the situation was "unbearable".
Ms Gregory raised her concerns with Labor candidate for Cronulla Teressa Farhart, who said the situation was "a disgrace" and "it is unacceptable for raw sewerage to be running into the local streets".
Ms Gregory said, "We have been living in Carabella Street for 45 years without a problem before this started happening".
"You cannot imagine what kinds of things are floating down our driveway and under our house - condoms, baby wipes, poo, all in a huge stream of sewerage water.
"We put this down to the overdevelopment in the shire and the fact that infrastructure has not been updated to cope with the extra sewer."
Ms Gregory said the situation worsened on Tuesday night.
"More neighbours are affected," she said on Wednesday.
"[Sydney Water] did a bit of a clean up yesterday but did not fix the problem. It is in fact worse.
"Tonnes of raw sewerage going down our driveway and under the house.
"Do we have to accept this gross contamination of our home space?"
Ms Gregory said the sewage overflow on Wednesday started at 4am.
"It is gushing down our driveway, our neighbours' driveways and now also the people who live behind us in Taronga Parade properties," she said.
Ms Gregory said her husband and Mr Badger had both called the emergency services at Sydney Water and spoke to the same person.
Neighbours in Taronga Parade had also called.
Ms Gregory said her husband and Mr Badger were told by the Sydney Water officer maintenance workers would laugh at him if he sent them out before 7am.
"We were told we should expect sewer overflows every time it rains, that the pipes are 100 years old and all the Water Board can do is provide a Band Aid solution," she said.
Labor candidate Teressa Farhart said the situation posed a threat to the health of residents as well as the general public because the overflows extended to streets.
"Local residents are being forced to guide school kids past their houses...to protect them from getting sick," she said.
"This is simply not good enough for the residents of Carabella Road.
"How long does it take for the current Liberal government to take action on this public health crisis and fix the local sewerage system?"
Sydney Water has been asked for a response.