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Inflow & Infiltration

What is Inflow & Infiltration?

Inflow is rainwater or groundwater entering the wastewater network via a range of defects on public and private property.

Typical inflow sources may consist of:

  • roof downpipes with incorrect connections to the gully trap.
  • low gully traps that act as low drainage points for stormwater.
  • damaged or improperly constructed gully traps.
  • at-surface manhole defects like holes in the lid or rim, particularly where surface ponding occurs.

Infiltration is when rainwater or groundwater drains into the ground and enters the wastewater network. Ways that this can happen include via:

  • cracked public sewer or private sewer pipes.
  • open and moved joints in either public or private sewer pipes.
  • cracks or construction joint leaks in manholes and other wastewater structures.

The wastewater network is designed to handle only the wastewater we all produce. Stormwater inputs limit how much our wastewater pipes can hold.  

If enough stormwater enters the wastewater network, it could cause issues like wastewater network overflows, sending wastewater onto people’s property and into the environment.  

It can also inundate or effect the wastewater treatment plants, which can further increase the number or volume of overflows in the wastewater network as well as reduce the level of treatment the wastewater receives before being discharged into the environment.   

Flooding the network with inflow and infiltration also increases the cost associated with wastewater pumping and treatment.  

This is not a new issue in Central Hawke’s Bay but it is something Council must address to reinforce durable infrastructure and allow for smart growth.

We have set aside funding in our Long Term Plan (LTP) to address this issue. In the current LTP period, we have been supported by Central Government by securing Central Government funding from the Three Waters Reform programme for this project. This has meant we have been able to invest more resources into investigation of our network. 

Other actions we have taken include: 

  • using CCTV cameras to identify any leaking pipes. 

  • We are monitoring flows and carrying out hydraulic modelling of the network to help us determine where stormwater comes from and how much flow there is. We are also investigating issues that arise on the network that can help pinpoint where stormwater is entering. 

Look at the stormwater and wastewater connections on your property. If you find any defects, there are steps you can take (outlined below) to fix them and ensure everything is running correctly. 


What the defect causes 

Steps to remedy the defect    

Downpipe connected to gully trap   

Stormwater will directly enter the wastewater network 

Connect downpipe to appropriate stormwater outlet    


Downpipe to ground    

Water ends up ponding on the ground and can end up under the house, resulting in rising dampness or scouring of your ground

Connect downpipe to appropriate stormwater outlet, not your gully trap    

Gully trap level with the ground    

Ponding water can overtop into the gully traps because the sides are not high enough    

Raise gully trap or put sealed barrier around the gully trap. Gully traps should be raised 25 mm above paved surfaces and 100 mm above unpaved surfaces. 
See NZ Building Code compliance regulations here.

Cracked or leaking gully trap   

Ponding water can get into the stormwater network via a property's gully trap

Replace the gully trap   

Terminal vent missing cover   

Vermin can get into your wastewater pipes and house as the terminal vent connects to the toilet   

Protect your terminal vent with a vent pipe cover   

No spouting or leaky spouting  

Water ends up ponding on the ground and can end up under the house, resulting in rising damp. The water can also end up getting into the pipework on your property if ponding occurs in the same area as the pipework 

Install spouting and downpipe, and connect to appropriate stormwater outlet    

Broken downpipe   

Allows stormwater to spill onto ground with the same impact as the downpipe to ground   

Fix the broken downpipe by connecting it to the stormwater network   

No back on gully trap    

Ponding water can get into the gully trap and have the same impact as a gully trap that is level with the ground   

Install a new concrete surround that is sealed 360 degrees in circumference or replace the gully trap   

Covered gully trap    

Covered gully traps mean sewage might back up into the house. The gully trap is an overflow point. If the lateral to the public network becomes blocked, the wastewater would normally come out the gully trap. By covering it, the wastewater would come out at the next lowest point on your property, which could be your shower or toilet   

Remove gully trap and cover with grating that will allow surcharge

Please let us know if you spot any overflows from the wastewater network by contacting Council on 06 857 806. Overflows can be caused by stormwater getting into the network or a system blockage, which requires immediate attention. 

Contact your local plumber or drainlayer if you find a defect on your property. They will determine if there is a problem on your property or in our network and advise of the relevant repairs. 

If you are unsure if you have a defect, you can send us a picture via our Send Snap Solve app and we can advise you. 

You can also contact our team at or call us on 06 857 8060. 


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