6. Improve overflows


Sydney’s sewer system is over 100 years old, and built for a smaller city. So, when it rains, stormwater can flow into the sewer system, causing it to overflow into our creeks and rivers. Understanding where, when and how this happens and putting measures in place to stop it are an essential part of making our river swimmable again.


Improve water quality outcomes through targeted wet weather overflow management, focused on those overflows impacting existing and proposed swimming sites.



Lead Agency

NSW Environment Protection Authority and Sydney Water.

Current measure

Overflows in the Parramatta River catchment meet all EPA licensing conditions.

Status Update

Sydney Water is working closely with the EPA to prioritise wet weather overflow abatement works in the Parramatta River catchment. Planned works are prioritised following a risk-based approach that considers multiple factors such as the frequency and volumes of Wet Weather Overflow discharges, the sensitivity value and use of the receiving environment and the cumulative impacts of multiple discharges.

The current priority is the Upper Parramatta Stage 1, which is a Source Control project. The assets are mainly Emergency Relief Structures (to manage and avoid heavy wet weather overflows discharging into customer properties or public areas). The design and delivery solutions included repairs of existing relief structures and new constructions, including concrete chambers, headwalls, pipes, duckbill valves, and in-line check valves.

Progress Timeline

April – June 2021

  • Stage 1 (inflow reduction) – designs for 81 Emergency Relief Structures (ERS) in the upper catchment completed and delivery of backflow prevention valves to 98 ERSs in the mid catchment is underway.
  • Stage 2 (infiltration reduction) – investigations and concept designs for 29 km sewer mains and 563 Maintenance Holes (MHs) is nearing completion.
  • Stage 3 (private properties) – inspections and smoke testing in upper catchment to commence in July 2021.
January – March 2021

  • Stage 1 (inflow reduction) works to the wastewater system are well advanced, with designs almost complete for the final stage in the upper catchment and construction works progressing in the mid-catchment.
  • Stage 2 (infiltration reduction) works, involving the CCTV inspection and relining of pipes are in design phase for the upper catchment. Works in the upper catchment will be completed by June 2022 and works in the mid-catchment to be completed by December 2022.
  • Sydney Water is finalising the planning for Stage 3 (illegal connections on private properties) works, with smoke testing expected to start in April 2021.
October – December 2020

  • Sydney Water business case/internal funding approved to review 98 sites and undertake repairs to prevent stormwater directly entering through emergency relief structures and manholes in the Mid Parramatta River catchment. Works are expected to be completed by January 2022.
  • Planning for program to smoke test and rectify illegal sewer connections on private properties in the Upper Parramatta River Catchment continuing. Works expected to commence in early 2021.
July – September 2020

  • Inflow reduction works to 47 assets have now been completed in the Upper Parramatta River Catchment. Modelling and gauging of these upgraded assets is now underway to understand benefits such as reduction in wastewater impacts.
  • A delivery business case/internal funding request for inflow reductions works in Mid Parramatta River Catchment is expected to be finalised by the end of September. This project will review 98 sites in the catchment with constructions works expected to be finalised by December 2021.
  • Stage 2 works (infiltration reduction) and Stage 3 works (works on private properties) are currently being scoped with commencement of both stages expected early in the new year.
  • The business case to resolve ongoing wastewater surcharge issues and high frequency wet weather overflows impacting Exile Bay and Hen and Chicken Bay has been approved. Works will deliver substantial water quality benefits to the new Bayview Park swimming site.
March – June 2020

  • Delivery in Upper Parramatta River catchment to be completed by 1 June. On 5 May, 43 sites were completed; two are in construction and two have not started (all of these in Baulkham Hills). Balance structures are expected to be completed by 30 June 2020. Once completed, modelling and gauging will be undertaken to measure the reduction in frequency and overflow volumes.
  • Middle Parramatta River overflows are in planning phase with initiation expected in July 2020.
January – March 2020

  • Delivery in progress.
  • 30 out of 47 overflow structures have been repaired/constructed in the Upper Parramatta catchment. Balance structures are expected to be completed by 30 June 2020.
October – December 2019

  • Delivery commenced.
  • 17 out of 47 overflow structures have been repaired in the Upper Parramatta catchment.
July – September 2019

  • Planning for Upper Parramatta River source control works program (inflow reduction) finalised and business case has been approved. Delivery is due to commence.

Useful Links

Review of Sydney Water Corporation’s sewage treatment Environment Protection Licences.