Access a range of published reports and testing results related to the Christchurch wastewater treatment plant.

We have undertaken an air monitoring programme, in order to:

  • Understand the gases contributing to the odour.
  • Identify the source of these gases (across the treatment plant site).
  • Measure the impact of the gases at various distances from the plant.

Odour and health

Since late April, and with the cooler temperatures, the oxidation ponds at the wastewater treatment plant have generated odour. The odour contains a mixture of highly odorous compounds - primarily hydrogen sulphide - that are odorous at much lower concentrations than the levels that can cause adverse human health effects. 

With the exception of hydrogen sulphide, the Council's community monitoring hasn't detected any odour-emitting chemicals at concentrations that could pose a risk to human health.  As a result, our monitoring programme now focuses on measuring hydrogen sulphide to see if levels in nearby residential areas are in concentrations and duration to potentially cause physiological symptoms.

Hydrogen sulphide is noted for its strong and offensive odour (like rotten eggs), even at low concentrations.

The point at which people can smell, and experience symptoms from, hydrogen sulphide varies between individuals, ranging from 0.0003 to 0.016 parts per million (ppm). The Ministry for the Environment’s air quality guideline for hydrogen sulphide is 0.005 ppm. This is based on preventing odour impacts on wellbeing rather than specific health effects.

The California Office of Environmental Health Assessment sets a guideline level of 0.03 ppm (averaged over an hour) to avoid symptoms such as nausea and headaches. At this level, 83 per cent of people will be able to detect the gas, with 40 per cent of them experiencing symptoms. These symptoms include nausea, headaches, eye and throat irritation, skin irritation, sleep disturbance and worsening asthma symptoms.

We have adopted 0.03 ppm as the “threshold for concern” for impacts on community well-being. However, the variation between people means there is a wide range of responses to hydrogen sulphide. People should be guided by their own symptoms when deciding whether to seek medical advice from their GP or healthcare provider.

While exposure to hydrogen sulphide at these concentrations is unpleasant and may result in health effects, it doesn’t accumulate in the body. Any effects would be expected to resolve when the odours are brought back to pre-fire levels, and long-term health effects are very unlikely.

Workplace standards for exposure to hydrogen sulphide are set at much higher levels to reflect the 40-hour per week exposure of a typically healthy adult population. The New Zealand Worksafe time-weighted average is 5 ppm (for 8 hour days and a 5 days working week) and the short-term exposure limit is 10 ppm (for 15 minutes).

Understanding the graphs below

Our monitors measure hydrogen sulphide in parts per million (ppm). You will see on the graph a purple dotted line going across 0.03 ppm. This is the California Office of Environmental Health guideline level, as mentioned above. Where you see a spike above this means that the levels for that hour (one spike = one hour) were higher than that level. The spikes are generally caused by sustained low to moderate wind flow in the direction where the monitor is located.

Since mid-June, we’ve had hydrogen sulphide monitors at three permanent locations surrounding the Christchurch wastewater treatment plant. These locations are – Bromley School, Marine Parade and Rudds Road.

The meters continuously monitor the levels of hydrogen sulphide in the air.

This data is displayed in a graph, one for each location. You can find the graph for your location of interest by selecting the relevant dropdown menu below.

A further three permanent monitors are being installed at Metro Place, Maces Road and Shortland Street.

Other monitors will be placed for weekly periods in other areas near the wastewater treatment plant to assess the levels and characterise the spread of odour.

Note: While the monitors work continuously, an air quality specialist downloads their data every Friday and then translates the information into graphs that are then sent to us. We aim to publish the previous week's data each Tuesday.

Published testing results and reports 

2 August 2022 - House discolouration investigation and findings

Christchurch City Council has received and published the findings of the houses tested for paint discolouration. 

Read the report [PDF, 4.2 MB].

All houses tested were found to have had lead-based paint, with almost two-thirds showing a likely reaction between hydrogen sulphide and lead in the paint.

The remaining houses didn’t show a chemical reaction, but our expert contractors, ENGEO, noted the discolouration could be removed by washing it with a chlorine solution, as the discolouration is most likely related to mould. This mould isn’t connected to the odour from the wastewater treatment plant, but is probably mould normally found on houses.

We have been advised that the discolouration is likely to only be temporary, and a 1966 study into the effects of hydrogen sulphide on lead-based paint does not suggest permanent damage to the paint. 

The discolouration is expected to gradually reduce as daylight hours lengthen, the health of the oxidation ponds improves, and we progress through spring and summer.

Any discolouration under the eaves could take longer to fade as it won’t receive direct sunlight.

What you can do if your house is affected


  • We expect the discolouration to fade as the health of the oxidation ponds improves, hydrogen sulphide levels drop and daylight hours lengthen.
  • Read the report findings and information
  • Wash your walls with a commercial chlorine-based solution, or warm, soapy water. This could help remove any organic dust, common moulds and some discolouration. The presence of these common moulds is not linked to the wastewater treatment plant fire.
  • Adhere to the manufacturers’ proprietary ratio recommendations if you use a commercial chlorine-based solution.
  • Wash your house several times if required.
  • If you're physically unable to clean your house or require assistance, we can provide advice and arrange help. Contact us at


  • Use a high-pressure washer to try and clean any staining or discolouration, as this could result in flakes of lead-based paint entering the environment.
  • Exceed the manufacturer’s proprietary ratio recommendations if you use a commercial chlorine-based solution.

 The Ministry of Health(external link) has information about lead-based paint.

A mail-out containing the above information and updates from the treatment plant site, and health information will be delivered to 6,700 homes – including South New Brighton and Southshore – from Friday. Due to the distribution area, some residents might not receive this mail-out until Monday.

June 2022

We've received our test results of the four houses that we sampled after concerns were raised about a black mould appearing on the exterior of some buildings near the wastewater treatment plant.

The results from a reputable external contractor have confirmed that the discolouration observed isn't due to mould. While sampling did detect the presence of various mould types, there were no spores or types outside of common mould that typically grows on the exterior of Canterbury houses. 

We're undertaking further investigations to determine what's causing the discolouration of external paint that some residents have reported. We will publish and share these results once we receive them.


This site is to the northeast of the ponds and relatively close to them. Winds from the southwest quarter carry H2S, and the odour, toward this location.

The graph shows that there have been no levels of H2S detected over the two weeks. During this time the wind has blown in the direction of this monitoring site. The fact that no H2S has been detected shows that at present there is little H2S being emitted from the ponds. 

This graph is displaying the hydrogen sulphide (H2S)results from monitors situated at:

  • Rudds Road, which will detect Hydrogen Sulphide carried on an easterly wind from the direction of the treatment ponds.
  • The Dog Shelter, which is located between Living Earth and the Oxidation ponds which will detect hydrogen sulphide coming from either the ponds or the estuary.
  • South Brighton school, which will detect hydrogen sulphide carried on a west or South West wind from either the ponds or the estuary.

The graph shows that only very low levels of H2S were detected on three occasions at the Rudds Road site. Much of this is likely to be from the estuary.

Until this week we have seen consistent trends of occasional H2S concentration readings at the sites near the estuary, coinciding with very low wind speeds and typically in the hours after low tide. 

2023 results

2022 results

This site is near the treatment plant end of Shortland Street. North-east winds will carry odour, including hydrogen sulphide, to this area.

This graph is for two weeks however no data is available for the first few days. For the remainder of the time only low levels of H2S were recorded. These may be from sporadic releases from tanks at the treatment plant, but at very low levels.

Monitoring at this site has been suspended while the monitor has been deployed to another site. Information about the South Brighton School site will still give an indication of likely levels of H2S in this area.

The site is to the east of the ponds and westerly winds will carry hydrogen sulphide (H2S )and odour to this site.

It is a representative site for houses that are further south on the spit. Typically readings here have followed a similar trend to those at the South Brighton school but any detected levels of H2S are low.

Monitoring at this site has been suspended in the meantime while the monitor has been deployed to another site.

Typically levels have been low due to the distance from the ponds and the effect of the wind.

For information on how the ponds are performing and H2S levels in the area, refer to the most recent graph of the four sentinel sites.

The site is to the southwest and some distance from the oxidation ponds.

Winds from the northeast would carry the odour and hydrogen sulphide (H2S) in this direction, but due to the site's distance from the ponds, the levels are more likely to be dispersed by winds.

Attached are graphs tracking the biological oxygen demand, suspended solids, ammoniacal nitrogen, faecal coliforms, and enterococci in the oxidation ponds for the Christchurch Wastewater Treatment Plant.