Untreated water leaked onto the capital's south coast beaches due to the Moa Point Treatment Plant flooding and being turned off. Photo: RNZ / Samuel Rillstone

A marine biologist says the impacts of Wellington's sewage overflow on a nearby marine reserve is depressing to witness.

About 70 million litres of untreated waste is now being pumped 1.8 kilometres out into Cook Strait after the Moa Point treatment plant failed last week.

The waste is now being screened for solid items like wet wipes and sanitary items but is otherwise raw sewage.

The capital's South Coast beaches are currently off limits because of contamination.

Dr Christopher Cornwall is a senior lecturer in Marine Biology at Te Herenga Waka-Victoria University of Wellington told Checkpoint that sewage was still potentially getting out onto the area's rocky reef ecosystems and the kelp forests around places like Taputeranga Marine Reserve and the rest of the South Coast, but also into some of the deep reefs.

"So that untreated sewage is still having ecological impacts, as to how big those impacts are we don't really know at the moment."

The South Coast marine reserve near where the sewage was being released was one of the best examples of marine protection in Aotearoa, he said.

"So we have things like increased numbers and sizes of pāua, koura, crayfish, we have things like a lot of fin fish species, so these will, towards the pipeline these will probably all be impacted.

"As to what those impacts are we're not 100 percent sure, they could range from small immeasurable impacts if this sewage pumping is stopped soon or they could range to larger ecological impacts such as the mortality of that kelp forest and the abalone that lives in it, the pāua."

Asked whether there were any historical examples that could give some indication of what might happen, Cornwall said that during the 1950s untreated sewage went out to a kelp forest off the California coastline and "it caused long term die-off of that kelp forest".

"In the 80s there was treated sewage that came out into that same area and it had lesser impacts."

In New Zealand, the treated sewage had measurable impacts on the seaweed community and potentially on the pāua at a Porirua reef although that data was still being analysed, he said.

Research that had been done at Porirua showed that there was a loss of some of the forest forming brown seaweeds or big kelp which were the habitats and food for a large range of shellfish and fin fish species, he said.

People snorkeling at Taputeranga Marine Reserve in May 2022. Photo: Kristine Zipfel

Seaweed takes up nitrogen naturally but things like the sewage outfall can cause this to become elevated, he said.

"That means the seaweeds taking advantage of that excess nitrogen and it can shift the balance in that seaweed ecosystem, so essentially it's taken up into their tissues and other nasties are also taken up into there and it can change the way that they function, for example it can cause some species that are invasive or opportunistic to increase their growth rates and out compete our native species."

The worst case scenario, which is less likely, would be the die-off of that kelp forest ecosystem - pāua, kina, koura and the fin fish species that live in that area, he said.

"We don't know at this point whether that will occur, it will depend on how long it takes for that sewage to actually start being treated."

Smaller impacts are more likely, he said.

"You can get things like green tide algae, so ulva, which will start out-competing some of the natives and increase."

That had happened in the past when there had been problems at Moa Point, he said.

If the sewage kept being pumped out it could also lead to things like harmful algal blooms, he said.

There was no way to mitigate the effects if sewage kept being pumped into the area, he said.

"The only way that we can actually mitigate this is by making sure that this doesn't happen in the first place and then secondly I guess we need to use this as an example of why we shouldn't be pumping our sewage out onto places like the South Coast, like the jewel in Wellington's crown as some people have put it recently. So we really need to think about using this as an example of what we shouldn't be doing for our wastewater management system."

It was depressing and disheartening to see sewage pumping out so close to the Taputeranga marine reserve, he said.

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