Reducing stormwater in the sewer

Reducing Stormwater in the sewer image_Banner_600x400.png

Stormwater or groundwater that incorrectly enters the sewer system is called inflow or infiltration.

On this page you will find more information about inflow and infiltration as well as information about reducing stormwater in the sewerage system in Mullumbimby.

What is inflow and infiltration?

What is inflow? 

Stormwater that enters directly into the sewer system.

Causes of inflow

Runoff from hard surfaces, such as roofs and driveways, which enters the sewer system through incorrectly installed downpipes, drains or low lying gully traps that overflow when it rains.

A gully trap prevents smells and overflows from entering buildings. They are usually located outside your house, generally in the area outside your kitchen, and should be above ground level to ensure that stormwater does not enter the sewer system.

What is infiltration?

Groundwater, or stormwater that seeps into the ground, then incorrectly enters the sewer system through defects in sewer pipes or joints.

Causes of Infiltration 

Cracked sewer pipes that have deteriorated from:

  • age
  • being placed under pressure from local development and heavy traffic
  • damage from tree roots
  • poor installation or maintenance.

Groundwater can then infiltrate through the cracks in the sewer pipes.

Infiltration usually occurs where the groundwater is higher than the sewer pipe or where soil above the sewer system becomes saturated from rain events.

Problems caused by inflow and infiltration

During heavy rain, excess stormwater that incorrectly enters the sewerage system can cause it to back up and overflow sewage into homes and the environment.

When stormwater or groundwater incorrectly enters the sewer system it can cause problems and increase costs for the community through:

  • Property damage: Flooding on private residences and businesses can occur as sewage pipes become overloaded with stormwater and groundwater and this does not drain away fast enough;
  • Environmental damage: The health of our waterways and its users can be compromised by overflow from the sewerage system when it enters our rivers and creeks;
  • Infrastructure damage: Roads and surfaces deteriorate with surface flooding and poor drainage, increasing maintenance costs for Council and ratepayers; and
  • Electricity and treatment costs at the sewage treatment plant increase to process the extra water in the sewerage system.

Reducing Stormwater in Sewer project

Council is committed to providing a sanitary sewerage system for the safety, health and well being of our community and conservation of our natural environment.

Council has embarked on the Reducing Stormwater in the Sewer project, which is looking at improvement to both the stormwater and sewerage networks. Read more about the project below. 

Aims of the project

  • To achieve a reduced level of stormwater entering the sewerage system that does not exceed the capacity of the sewerage network.
  • An improved sewerage and stormwater network to better accommodate the area’s growing population and more extreme weather in the future.
  • Undertake flow monitoring of the sewerage network to better understand how it operates during rainfall events.
  • Inspection of Council’s sewer and stormwater mains and residential sideline connections to assist in identifying problem area.
  • Undertake rectification works, such as those now in progress in Mullumbimby, and further visual assessment (CCTV) and smoke testing, which are being carried out in Byron Bay and Ocean Shores.

The program has received funding of $5 million over five years, with $2.5 million allocated towards the worst affected area (Mullumbimby).  




  • The efficiency of the sewerage network will be improved as the identification of defected pipes will prompt targeted repairs. 
  • Costs of damage to personal property and homes will be reduced as efficient plumbing systems and sideline connections will reduce the risk of stormwater overflow and localised flooding.
  • The risk of wastewater overflow into residential properties and street frontages will be reduced. 
  • Electricity and maintenance costs of sewage pumping stations will reduce.
  • Risk of pollution to local rivers and creeks will reduce and the biodiversity and health of the waterways and its users will be protected. 
  • Street aesthetics and stormwater drainage in Mullumbimby will be improved. 
  • Reducing water waste and using water more efficiently will help protect Mullumbimby’s water source for the future.

Mullumbimby is the worst affected area in the Byron Shire and experiences a significantly high level of stormwater infiltration and inflow into the sewer during rainfall events. The CBD area in Mullumbimby in particular is largely affected.

Mullumbimby is a low lying, flood prone area that is adjacent to a river and receives a high annual rainfall.

In a storm or rain event, the intensity of water is high and quick. In low lying areas, the stormwater gathers on the surface and can enter the sewer system rather than draining into the stormwater system. Inflow and infiltration has been a recurring problem for Mullumbimby since the 1960s, when the sewerage network was first constructed.

Other areas of the Byron Shire will also receive assistance with the reduction of stormwater into the sewer. The investigation stage has commenced in these areas.

Inspections of Council’s stormwater and sewer mains in the Mullumbimby CBD area were done in 2019 by authorised specialist contractor, Interflow.

Council are currently undertaking assessments within certain catchments in Byron Bay and Ocean Shores that have been identified as high risk, based on various criteria including age, material type, depth, etc.

What was involved in the inspections? 

Council and specialist contractor Interflow used a variety of inspection methods to identify sources of stormwater inflow and infiltration into the sewer in Mullumbimby.

Standard inspection methods included camera and visual inspection, and non-toxic smoke testing.


Smoke testing involves the pumping of non-toxic, non-staining coloured smoke into a public or private sewer or manhole during dry weather.

Possible sources of stormwater inflow, infiltration or cracked pipes are identified at points where the smoke becomes visible, such as from yards, roof gutters or pipes.

Camera inspections involve the insertion of a robotic vehicle with a mounted camera into the sewer main through access points such as maintenance holes. The main is typically cleaned via high pressure water blasting prior to recording the sewer main to ensure that all defects can be easily identified.

Pumping station systems for Mullumbimby were upgraded in 2009-2010, which have contributed to resolving the majority of sewage overflow or septic backup in residential areas, however significant inflow and infiltration is still occurring.

Flow monitoring

 Flow meters were installed into the sewer network with drawdown tests and ultrasonic level sensors, as well as rain gauges and weather stations, which recorded and provided data on the volume of flow being transported through the sewer catchment.


 Inspections of Council mains have been carried out with specialist contractor, Interflow, conducting smoke testing and camera inspections in the Mullumbimby CBD area. These inspections are continuing in other areas of the Byron Shire, such as Byron Bay and Ocean Shores.


 Modelling of the stormwater network within the Mullumbimby CBD area will be undertaken prior to June 30 2020 to ensure stormwater mains that have been identified to be in poor condition will be replaced with pipes that account for the future growth of the catchment.


Rectification works on the gravity sewer network are currently underway in Mullumbimby’s CBD area. The highest risk assets, as identified by the assessments that were undertaken in mid-2019, are being prioritised.

A good sewerage network requires a combined approach from Council and community.

Property owners might not always be aware that rainwater from their property is entering the sewerage system. Previous residents may have connected a rainwater pipe illegally to the network.

Simple checks and prompt action might make a huge difference to our community’s sewer and stormwater systems.  

  • Replace old leaking sewer pipes from your house to your property boundary;
  • Redirect your stormwater away from the gully trap; and
  • Replace any broken or leaking gully traps.

For any sewer issues on your property please call Council's 24 hour number: 02 6622 7022