Inflow and infiltration


drain smart logo web 210 208As part of the Long Term Plan 2018-28 Council adopted a vision for Nelson, the smart little city. The first part of that vision states that Nelson is a vibrant place where we are deeply connected with, and committed to, our natural, social and cultural environment.

Our environment is particularly close to our hearts. As a coastal city, we are lucky to have some beautiful beaches and rivers that are enjoyed by young and old alike. We all want to protect these special places.

Historically, like many cities, Nelson has experienced wastewater overflows when heavy rain hits our region. This has resulted in wastewater getting into our waterways, which has a detrimental effect on our ecosystems and is frankly just not very pleasant for anyone.

This isn’t a new issue and it’s not just happening in Nelson, it happens all over the world, but it is something that Council has been working very hard to address. 

What is it?

Inflow and Infiltration (I&I) refers to rainwater and groundwater that enters the wastewater system through a variety of defects on public and private property.

Inflow sources allow rainwater to enter the wastewater system directly from the surface through incorrect plumbing, cross connections and damaged or low-lying gully traps or manholes.

Infiltration sources allow the groundwater to seep into the wastewater system through cracks or bad joints in wastewater pipes and manholes.

A certain amount of I&I is unavoidable and it is planned for in routine wastewater design.  But too much I&I, especially during severe weather events, can overwhelm the wastewater system, leading to overflows and the associated risks to health and damage to the environment.

The increased frequency of extreme weather events and overall growth in our region is leading to the risk of more overflows. This means we need to start taking wider action.

Council has prioritised making further improvements on this issue and has allocated funding in the Long Term Plan for inspections on private properties to identify any I&I issues. This will give a fuller picture of the extent of the problem and Council will be better informed to develop a strategy to reduce it.

Evidence from other towns and cities shows that this is unlikely to be a quick and easy fix. It will take many years to fully understand and address I&I in our city and finding solutions will be critical.

Inflow – here’s what happens

We’ve put together a short video that explains the basics about inflow and how you can do your bit to help reduce it.  Check it out here.

What is Council doing?

Work has been happening to address I&I for some time. Council is inspecting its own properties for any issues and fixing them as needed. There is also an extensive programme of renewals and repairs to the piped network.

The current emphasis is on renewing earthenware pipes that are 80-100 years old and those that are in areas with high levels of groundwater. Targeting these pipes helps reduce the amount of groundwater that enters the pipes through cracks, joints and faulty manholes.

Inspections of privately owned properties have been done throughout the city, looking for issues that contribute to I&I. This fact-finding is crucial to give a clear picture of the issue and enable Council to make better decisions on how to proceed. If Nelmac come to your property they will also leave some information showing what to look for and explaining any issues on your property.

How can I help?

As a property owner, you play an important role in addressing this issue and helping to reduce the risk of overflows.  If you find any issues that you can easily resolve, please take action now. You'll be part of the solution, not part of the problem. 

Please go outside your property and check these three things; downpipes, gully traps and sumps.


Look at your roof, where do your downpipes go?  They should connect to the stormwater system, not the sewer/wastewater system. If the downpipes from your roof connect into a gully trap, then you will need to re-direct it into the stormwater system.

downpipes the right and wrong

What is a gully trap?

A gully trap is a plumbing feature that should only receive wastewater from your kitchen, bathroom and laundry. The gully trap connects to the sewer (wastewater) network which takes wastewater to the treatment plant for treatment.  The top or surround of the gully trap should be above ground level and partially covered to stop stormwater/rainwater and other foreign matter (such as landscaping bark)  entering the wastewater network.

What should my gully trap look like?

gully traps the right and wrong
A good gully trap has covers and a high surround that stops rainwater going into the wastewater system during periods of heavy rainfall

A faulty gully trap will let rainwater into the sewer/wastewater system.

What does the building code say about gully traps?

This diagram of a gully trap is from the New Zealand Building Code. The surrounds of the gully trap have to be 25mm above a paved surface or 100mm above an unpaved surface.

Gully trap


sumpA sump is a stormwater feature that collects rainwater from external surfaces such as driveways and patios.  The sump has to connect to the stormwater system, not the sewer/wastewater system.

The only way to test for a cross connection without calling a plumber or drain layer is to check for a foul odour that is stronger than a normal organic /vegetation smell.


If you have questions or concerns please contact Nelson City Council on 03 546 0200.