A sewage spill at Kowhai Park in 2015. Photo / Bevan Conley
Whanganui District Council could face a $50 million bill over the next 30 years to address its “swamped” wastewater network.
A report from the council’s senior stormwater engineer, Kritzo Venter, says a substantial amount of stormwater is still being disposed of via the wastewater system.
While the network performed adequately during dry-weather flow conditions, modelling showed it often “lost containment of diluted wastewater” during rain events.
Venter told a council meeting on Thursday that the wastewater system was breaching through access chambers where lids popped.
“In the catchment in Whanganui, if there’s a surcharge from diluted wastewater, it ends up in the stormwater [system] and inevitably ends up in the Whanganui River.
“It’s even coming out of low spots like people’s gully traps.”
A gully trap is a basin in the ground that receives piped wastewater from a building before it enters an underground wastewater network.
Directing stormwater down a gully trap was a major issue, and management of non-compliant connections was currently non-existent, Venter said.
“There is no firm compliance framework in place.
“We need to have the community on board to solve this issue.
“It’s about the environmental impact it has, from a Te Awa Tupua values point of view, and also, it’s a prohibited activity to spill any wastewater into the environment.”
Proposals in the report include capital investment of $50m over the next 30 years and inflow and infiltration investigations and reduction at $200,000 per year.
Capital works include pipe upgrades, pump station upgrades and new wet-weather storage facilities.
The council is already undertaking a major wastewater project in Whanganui East - replacing a 113-year-old, 911-metre pipe that runs from Anzac Parade to the five-road junction where Raine Street, Wakefield Street, Nixon Street, Burton Avenue and Duncan Street intersect.
Stage one, which covers 549 metres and is expected to be finished in October, will cost about $2.4m.
Venter said the council’s current treated wastewater discharge consent expired in 2026.
“An application will be submitted to Horizons [Regional Council] by around November 2025. That sounds like a lot of time, but it is not, really.
“We have to recognise these issues openly so a consent application can be properly informed.”
Council chief executive David Langford told the Chronicle the council had to come “face-to-face” with the issue.
He said overflows were occurring multiple times a year when there were rain events.
The report highlighted previous surcharge incidents at Kowhai Park and Moutoa Quay.
Both required de-contamination procedures.
“When this happens, our team goes down, we clean up and disinfect the whole area and make sure it’s safe,” Langford said.
“But it shouldn’t be happening in the first place.”
He said it was important to be honest with the community about the reality of the situation.
“We have to fix this problem, and we’re going to have to spend some money to do it.”
Langford said it wouldn’t be as easy as building more and bigger pipes because illegal connections were a big factor as well.
Identifying them was done by a smoke detection programme, which would now “ramp up”.
“Basically, you find an access chamber in the network and you pump smoke into the pipes,” Langford said.
“You look for where it comes out in private properties.
“If you can see it coming out of stormwater gully traps or roof gutters, then you know there’s a connection where stormwater is getting in.”
Enforcement action could then be taken if required.
Langford said the wastewater system in upper Aramoho, St Johns Hill and Westmere was functioning well, but that was to be expected as those areas were at the very top of the network.
Things changed when the pipes reached closer to the city’s wastewater treatment plant, near the airport.
“By then, most of the city’s wastewater is flowing through one or two pipes.”
Venter said Ngā Tāngata Tiaki o Whanganui was very supportive of the fact the council was prepared to address the issue through its Long Term Plan.
“Basically, the message we’ve heard is, ‘Just get on with it’.
Langford said if the river was unhealthy and contaminated with sewage, people would struggle to be healthy as well.
Like Venter, he emphasised the importance of honouring the values in Te Awa Tupua and looking after the river.
“You wouldn’t want to go for a swim or fall out of a canoe whilst you’re on the river if waste is floating around.
“People are pulling fish out of the river if they’re fishing off the new fishing platforms.”
It was currently unclear how many private properties were non-compliant.
Venter said a small audit on a catchment that met separation rules in the past had a 15 per cent non-compliance rate.
“The steps of getting people to comply, and the costs of that, haven’t been estimated.”
He said proposed budgets had been shared with the Department of Internal Affairs in preparation for a possible future transfer to the new Three Waters entity.
“At this point, the next two to three years or possibly even longer may have to be committed to by the council.”
Speaking at the meeting, Langford said $50m could sound quite alarming for the community.
“If you spread that evenly over 30 years, you’re talking $1.6m a year.
“We already spend $18 to $20m a year on capital projects just for Three Waters.
“There will be a lot of efficiencies we can deliver by doing some of the additional work at the same time as replacing old pipes that we needed to spend money on anyway.”
Mike Tweed is an assistant news director and multimedia journalist at the Whanganui Chronicle. Since starting in March 2020, he has dabbled in everything from sport to music. At present his focus is local government, primarily the Whanganui District Council.
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