Published reports and testing results related to the Christchurch Wastewater Treatment Plant.

We have strategically set up hydrogen sulphide (H2S) monitors at locations around the oxidation ponds and nearby communities. These have been in place since July 2023 and the results are regularly published on this page. 

We have undertaken an air monitoring programme 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.

Our monitors measure hydrogen sulphide in parts per million (ppm).  The meters continuously monitor the levels of hydrogen sulphide in the air. 

Latest hydrogen sulphide levels

We are continuing to monitor levels of hydrogen sulphide at residential locations around the wastewater treatment plant. Monitoring results have shown that levels over the past month have been decreasing and are now well below the health guideline of 0.03ppm. At times light winds still carry hydrogen sulphide, and other odours, to nearby residential areas but the levels are below 0.01pmm.

Odour and health

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% of people will detect the gas, with 40% experiencing symptoms. These symptoms include nausea, headaches, eye and throat irritation, skin irritation, sleep disturbance and worsening asthma symptoms.

You will see on the graph a purple dotted line going across 0.03 ppm. 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.

Where you see a spike above this it means that the levels for that hour (one spike = one hour) were higher than that. The spikes are generally caused by sustained low to moderate wind flow in the direction where the monitor is located.

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 five ppm (for eight-hour days and a five-day working week) and the short-term exposure limit is 10 ppm (for 15 minutes).

Previous testing results and report

2023 results

Combined air monitoring results

2022 results

Combined air monitoring results

Bromley School air monitoring results

South Brighton air monitoring results

Rudds Road air monitoring results

Shortland Street air monitoring results

Estuary Road South air monitoring results

Maces Road air monitoring results

House testing results

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

What you should not do:

  • 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.

Water quality testing results

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. 

Initial air quality testing results