The Northern Suburbs Ocean Outfall Sewer, commonly known as the NSOOS, extends from Blacktown to North Head. The 3.5-meter-high, 2.6-meter-wide tunnel runs along the north side of the Parramatta River and was constructed between 1916 and 1933, mostly through deep underground rock. When it was built, concrete was used to line the NSOOS and protect the rock from corrosion caused by wastewater and sewage.

This 28km-long NSOOS collects around 25% of Sydney’s total sewage each day.

The concrete lining has now deteriorated, resulting in large pieces falling from the roof of the sewer tunnel and it is now sitting on the tunnel floor, collecting further silt and debris (including poo, wet wipes and any number of other things put down the toilets of Sydney) reducing the capacity and efficiency of the system. Work inside the sewer to restore it to its optimum condition has been ongoing over the past few years. Sydney Water has developed custom made machines that can be driven through the wastewater tunnel by specialist workers to remove the debris. New spray-on abrasion resistant concrete has been applied to the roof and walls.

Since 2017 Sydney Water has de-silted and rehabilitated approximately 10 km of the tunnel between Lane Cove and Cammeray and in 2024 the Cremorne to the Spit section was completed. Overflows at the Spit during an extreme wet weather event caused delays as the priority shifted to resolve these pollution issues in Middle harbour near Clontarf. The final section of the NSOOS to be de-silted is the section from Clontarf to North Head.

While this rehabilitation work is underway, some of the wastewater will be redirected to the Northside Storage Tunnel (NST). But parts of this tunnel also need to be cleaned out underground at the North Head facility so it can continue operating successfully as a diversion to the NSOOS to prevent overflows into the harbour. Work to clean this tunnel has been occurring over recent months. All the silt and debris removed is then processed at the treatment plant at North Head, where the sludge is dried and removed as bio solids in trucks to be composted and processed for use in agriculture.

The teams doing this dirty but essential work will be accessing the NSOOS via several locations along Darley Rd. Sydney Water will be excavating a cavern at the treatment plant for the floating de silting digger machinery to be lowered 90m to the tunnel. Access points are also planned for Bluefish Rd just past the pedestrian crossing on the left side, opposite Manly Hospital and at the bottom of Darley Rd.