Orakei sewer collapse– is what is happening at Taipari Strand worse?

Manhole 10 is a manhole on the Whenuapai transmission sewer network, located at Taipari Strand. This transmission line conveys sewage from local sewage networks (Te Atātu Peninsula) and upstream points in the Upper Harbour west catchment to the Māngere waste water plant. Manhole 10 is a low lying manhole identified by Watercare with capacity unable to cope with the volume flowing through it during heavy rain. This makes MH10 very prone to overflows, and during heavy rain (over 30mm in a 24 hour period) MH10 will generally overflow sewage into Te Wai ō Pareira. 

Watercare performance reports record MH10 as the worst overflowing manhole in the entire Auckland transmission network yet there are no short term plans to improve it. There are plans for other parts of the network that should bring some future reduced frequency to the overflows at MH10 but not eliminate them.  A recent Watercare report to the Henderson Massey Local Board includes modelled estimates of 6 overflows at MH10 per year in 2030.

This is a much higher number than is allowed under Watercare’s network discharge consent for Engineered Overflow Points (EOPs), with these, Watercare’s resource consent sets a target of 2 overflows per year.  For persistently overflowing EOPs the consent manager (Auckland Council) may allow a higher target.  MH10 is not and cannot be upgraded to an EOP and thus slips through the cracks and frequency targets are not set by the discharge consent conditions i.e. there are no regulatory controls over waste water discharges to Te Wai ō Pareira.

Let’s compare what is happening in Te Atatū with what we saw during the Ōrākei main sewer collapse in Parnell, 2023. That was a bad situation with a total collapse of the pipe leading to raw sewage flowing directly into the harbour and this happened for 20 days before the bypass was placed. 20 days was an incredible effort and a great response considering the scale of the collapse and the complication of working with 100 year old pipes. The ongoing remedial work has been intensive and involved relining a 1.6 km section of pipe to prevent further breakages and the entire project has cost about $86 million.

What is happening at MH10 is not as dramatic as the sewer collapse in Parnell but like that event there is sewage directly entering the harbour.  Unlike Ōrākei it is ongoing and it will not stop. There were 34 overflows in the last 3 years at MH10 and there is no direct remediation work planned for this manhole, no $86 million project to prevent sewage entering the harbour or even a $1 million project. In fact when there are overflows at MH10 there is barely a response from Watercare, they send a clean up crew after the overflow but there is nothing further done to prevent the recurrence.  It is noted that the Northern Interceptor, when completed, may reduce the overflow frequency at MH10, but in our opinion will not eliminate them.  

Looking at the numbers of the fecal indicator bacteria Entrocci is very interesting During the initial Ōrākei event the numbers were 60,000 mpn (most probable number) which is incredibly high but numbers were down to 17,000 mpn in the following weeks and this event is now over and the numbers are now generally  within the safe range of <280 mpn But for Te Wai ō Pareira the water can test as high as 24,000 mpn following an overflow and this is going to keep occurring. 

Every time MH10 overflows, the water becomes unsafe for at least 48 hours following the overflow. Currently there are an average of 11 overflows at MH10 per year. In the last year there have been about 26 days where MH10 has been overflowing and this has resulted in at least 50 days or 14% of the year where the water has been dangerous to humans, dogs and wildlife. Add in the data from all the overflows that occur from other manholes and there is at least 70 days where the awa is not safe, that is far too many for a highly utilised area with an important ecological function.

We argue that the ongoing risk of what is happening to Te Wai ō Pareira is greater than the single event at Parnell. We want to see a response to overflows similar to what we saw for the Ōrākei event and for overflows at MH10 to be given urgency. Te Wai ō Pareira is a highly utilised waterway, enjoyed by many for recreational activities and is an important ecological area, supporting a huge variety of native species. According to Watercare when there is a high volume of sewage discharged into a sensitive environment this triggers  a high response level and requires a specialist response, such as what we saw in Parnell. Unfortunately MH10, not being controlled under the network discharge consent, allows the discharges to occur without that specialised response even though Te Wai ō Pareira is a sensitive environment and the volume of discharge is high. We do not feel this is acceptable and ask Watercare to take what is happening here in West Auckland as seriously as it did for Central Auckland.