Wellington Scoop

More sewage discharged into Titahi Bay, after heavy rain

News from Wellington Water
A wastewater bypass discharge has occurred from the discharge point at Rukutane Point in Titahi Bay – it may affect Titahi Bay and associated shellfish gathering areas near the discharge point.

From time to time there is a need to discharge some partially treated (screened) wastewater along with the fully treated wastewater, in particular when there is heavy rain and there is too much water in the treatment plant.

A partially treated discharge is when a small portion of the wastewater ‘skips’ a component of the treatment process. This partially treated wastewater is mixed with the fully treated water and the public health risk from these discharges is very small.

But as per Regional Public Health guidelines, swimming or aquatic activities are not recommended for 48 hours after heavy rain.

As the city has grown, the frequency of these discharges has increased. The planned upgrades to the Porirua Wastewater Treatment Plant will address this issue.

Campaigner Michelle Laurenson of Your Bay Your Say comments:

In heavy weather events (as just experienced), the flow of wastewater coming into the Porirua wastewater treatment plant can increase to 300 times what it can cope with (Three Waters Strategy-2018, pg 20). Currently the plant bypasses at 950L/s and UV treatment at 980L/s (AEE, April 2021) and in 2023 is supposed to handle 1500L/s – which will still bypass with flows during heavy rain events still reaching 3000L/s-5000L/s. Proposed storage tanks and upgrades to pumps to help stem these flows are not guaranteed to stem bypassing at the plant, but will help overflows into Porirua Harbour.

When it bypasses, the plant then releases poor quality effluent – “enterococci levels coming into the plant and bypassing out, in wet weather events can be 50 times more than accepted levels for water quality criteria” (pg 5 Appendix N: Virus Reduction Performance and pg. 15 of AEE revised information for consent application). Also the Porirua plant does not sample until 24 hours after these events (pg 20 of AEE revised information for consent application) – so the statement that “the public health risk is very small” is not correct, because they haven’t sampled yet.

Plus in the report Porirua WWTP-Process Model Ammonia Concentrations – 28 January 2021 pgs 4-8 in the proposed application for re-consent – it states “there is an absence of long-term data for inflow and effluent quality” and “there is no long-term effluent ammonia data for the plant”. Given that their projected population is significantly under-estimated (by at least 20,000) and they mention ammonia levels will rise with increased population, this is very concerning, as there are incidents of bowel cancer connected to high levels of ammonia in water.

Also mentioned on pg 8 of this report is “currently there is significant un-aerated volume (65%) for the Porirua Treatment Plant”; this is the secondary treatment where faecal bacteria is removed, and UV disinfection does not work on such poor quality effluent. The plant bypasses poorly treated sewage to discharge, and this further proves that the notification is not “a small risk to the public” .

Failure of UV system discharges sewage
Second failure of UV system

August: Ten sewage overflows into Porirua Harbour
November: Raw sewage on Titahi Bay beach
January: Sewage on beach “heartbreaking”
February: Communication failures after sewage overflows
March: Community angry about sewage overflows
April: Second failure at sewage treatment plant


  1. Ange, 20. June 2021, 15:11

    Michelle, thank you for illuminating these facts and for providing references so we can verify the information ourselves. Is it usual practice at other plants for waterways not to be tested till 24 hours after overflows have occurred ?

  2. Michelle Laurenson, 20. June 2021, 20:33

    Compared to the other plants in the Wellington Area and particularly Moa Point, it is not usual, as discharges (which bring in high levels of contamination) need to be sampled within 2 hours of a discharge.

    Independent reviewers Jacob Engineers (Section 3.3.3 Bypass Event Monitoring) critiqued the consent application – saying monitoring taken 24 hours after the discharge event and thus 2 tidal cycles was limited and that sampling at the seven monitoring sites for these events would be unknown.

  3. Michelle Laurenson, 22. June 2021, 7:57

    Another discharge bypass occurred yesterday 21 June, from Porirua Treatment Plant.
    The new consent (prepared by Wellington Water) proposes to stop monitoring all bypasses by 2023, and with effluent so poor when these bypasses occur, and population is going to significantly increase in the catchment – it is alarming that such a proposal would be made.

    It certainly appears this proposed consent is a document trying to cover-up an obvious need for more investment for the ageing Porirua Treatment Plant.

    Why put money in when you can manipulate s resource consent to gain a license to pollute.

  4. Barbara, 26. June 2021, 13:30

    There is a brown discharge into the sea at Rukutane Point at noon TODAY. The weather is rough – although the surfies are out in it at TIB beach – but the rain surely hasn’t been heavy-enough to cause another bypass?