Meola creek would not look out of place in the countryside, but every year, one billion litres of sewage-tainted water rushes down its length.

There's a little creek running through suburban Auckland, a decent stride wide and water shin deep, that moonlights as one of the country's biggest drains.

Not so long ago, it was called Waititiko, 'water of the periwinkles;' now, it's a regular conduit for raw human sewage, and a living illustration of the city's complicated relationship with waste.

Unlike many waterways, Meola Creek, as it is more commonly known, is at its worst at its headwaters. The creek first appears on land somewhere north of Mount Eden, where it is almost immediately tainted with wastewater; one of the city's largest sewer outfalls, on the grounds of Plant & Food Research, pours a mixture of stormwater and wastewater into the top of the creek.

Meola vanishes underground from there. It was artificially piped a few decades ago, and resurfaces at a graffiti stained culvert a few hundred metres north. The shadow of where the creek once flowed is imprinted on the ground in a weedy, green corridor. The culvert is blocked by a solid metal grate which looks immovable, but when there's a storm, it is pushed upwards, horizontally, by the cascade of water.

READ MORE:

* Rivers must be protected for future generations

* Popular swimming spots are declared 'no-go zones'

* $1.2b Central Interceptor wastewater tunnel to keep Auckland beaches clean

* Sewage spews into Auckland harbour; Watercare to investigate illegal downpipes

* Sewage overflow could be moved from Manukau to Waitemata

Every weekday afternoon, students from Mt Albert Grammar stream over a bridge into a small reserve a bit further along the creek. On a day earlier this summer, the creek, even from the bridge, has the faint smell of decay.

Many of the students pass one of several manholes, which, during a storm, can be lifted upwards by the flow of water, spilling water into the reserve. There are many of these manholes – they are technically known as engineered sewage overflow points – along the creek, and their purpose is to release the build-up of water pressure in the network. The result is a soupy mixture of stormwater and sewage spilling over the grass and into the trees, leaving tissues and tampons and condoms strewn in the leaves.

One of the manholes that spills wastewater is around 10m from a state house, which property records show was built in 2008, long after it was known that it would share a backyard with the occasional sewage overflow.

ABIGAIL DOUGHERTY/STUFF Meola Creek is usually a trickle, but when it rains, it flows with wastewater, part of which is raw sewage.

Every year, around two billion litres of this sewage-inflicted water ends up in the Waitematā Harbour. About half of that comes from the tiny Meola Creek.

There are more than 200 overflow points across Auckland, each of which has between two and 52 spills a year. The spills are most common along Meola, where they happen virtually every time it rains. Once the cocktail of water, sewage, and other debris enters the creek, it spills out onto the Meola Reef, a buried lava flow in the harbour, and drifts to the beaches dotting the nearby suburbs.

It is common to see these beaches stamped with health warnings after it rains, and this is why; raw sewage from the inner western suburbs is ushered into a creek, left for the harbour to handle.

Safeswim, the region's water quality monitoring programme, tracks 84 of the city's beaches; after heavy rain, as many as 50 can be considered to pose a risk to human health (on a sunny day early this month, fewer than 10 beaches had health warnings).

What makes Meola uniquely susceptible to overflows is the surrounding infrastructure.

Around 4 per cent of Auckland does not have a separated wastewater and stormwater network, a relic of early 20th century design. The pipes can handle the everyday demands of wastewater - the toilet flushes, the shower water - but when it rains, the influx of stormwater overloads the system. The pressure builds, and if the water is not released proactively, it risks flooding roads and businesses and houses with sewage-tainted water.

In the western isthmus, the philosophy is to get rid of this water as quickly as possible. Meola Creek is a testament to this idea; a gentle stream that would look perfectly natural in the countryside, a habitat for pukeko and tuna and whitebait, mangled along its length with heavily engineered outfalls, pipes and culverts.

ABIGAIL DOUGHERTY/STUFF Wastewater overflows streaming into Meola Creek through an outfall at Lyon Ave, near the mall at St Lukes.

Further downstream, Meola continues its slow sashay beneath the giant screens that catch rubbish and underneath pipes that spray stormwater into the creek. Throughout, it is fed by small, clear springs, pumping clean groundwater into the polluted creek.

These springs are what made Meola a good candidate for flushing the city's waste: "Most creeks get dirtier and dirtier as they go towards the sea," says Liz Walker of the St Lukes Environmental Protection Society, a community group dedicated to cleaning up the creek.

"What they did with Meola is say, 'great, it's filled with these springs and this groundwater, so let's just pour all of our s... down that creek because it will get diluted.'"

This may have worked when Auckland was smaller and less urbanised. Today, the sheer amount of water that bounces off the ground is too much to handle.

LAWRENCE SMITH/STUFF Liz Walker, resident in Mount Albert, Auckland, runs a local group advocating for the clean up of the Meola Creek.

For people like Walker who live near the creek, the regular overflows have become untenable. Early in 2017, then local MP Jacinda Ardern posted a video on her Facebook page after meeting with Walker and her group. Ardern speaks to the camera in front of a sewage outfall behind the St Lukes mall, one of the single biggest contributors of sewage into Meola, and likely among the biggest overflow points in the country.

"It just demonstrates how ill-equipped we are for weather events when it comes to sewage management in Central Auckland and Mt Albert," she says in the video.

She proceeds further up the walkway, continuing to speak to the camera: "I can see, with my naked eye, right along this stream, condoms strewn around from when we've had heavy rain events," she says.

"We need to clean this up."

ST LUKES ENVIRONMENTAL PROTECTION SOCIETY/SUPPLIED A massive overflow in Meola Creek during heavy rain. The water is contaminated with sewage.

At the bottom of Meola Creek, where it dissolves into the Waitematā harbour, there's a dog park.

"I quite frequently get phone calls from members of the public that their dogs have got giardia," says Nick Vigar, manager of the Safeswim programme.

"Giardia can come from a number of sources in a rural context, but in an urban context, there's really only one source – human wastewater."

The health impacts of poor water quality are notoriously difficult to quantify. It can take up to 10 days for a waterborne illness to exhibit symptoms in humans, and even then, only a tiny minority will go to a doctor.

LAWRENCE SMITH/STUFF Meola Creek is heavily modified with culverts and outfalls.

"We know when it comes from a human source, your chance of getting some sort of gastroenteritis, some bug, are that much higher," Vigar says.

"There's a whole lot of international evidence to that effect, but there's also a lot of anecdotal evidence, particularly around the [western isthmus], where we know the bulk of that water is coming from human overflows.

"I'm not in any doubt that the risk, particularly in beaches in those areas, is particularly high."

The severity of the pollution can be extreme, particularly during wet weather overflows. Testing of water in Meola Creek in 2011 showed E. coli levels of 2,400,000 cfu/100ml, more than 4000 times the guideline level. Around the same time, tests from one part of the creek in both wet and dry conditions contained Norovirus, which is highly contagious and potentially life-threatening.

ABIGAIL DOUGHERTY/STUFF The creek's water takes on a glassy colour.

A few years ago, Auckland changed the way it reported its water quality, which brought this long-ignored problem into the light.

Health warnings used to be issued after physical samples were tested, which could take up to 48 hours, by which point the pollution was likely gone. Those samples were regularly collected via helicopter, which were only able to fly in calm conditions, when the water was least likely to be polluted.

A 2016 review found this system had "consistently underestimated the frequency of contamination events".

That may have been an understatement: The review said over 22 years, just one contamination event had been noted at Red Beach, near Whangaparaoa. Newer testing in 2017 found four such contaminations at the beach in a single day.

A recent analysis estimated the previous method was recording around 4 per cent of contamination events. The new one – which is based on modelling, rather than retrospective testing – was recording between 67 and 86 per cent of contaminations.

It led to a sharp increase in the number of contamination events being reported; not necessarily because they were new, but because they were no longer being missed.

"I think it's been a bit of a shock for people," Vigar says.

"The beaches are no worse now than they were five years ago, but the difference is it's very visible to people now."

For an issue that took many decades to develop, the solution – or a major part of it, at least – is on the horizon. It is called the Central Interceptor, a massive pipe that will transport water from the western isthmus to the city's wastewater treatment plant in Mangere.

It will be a pipe big enough to drive through, which, at its deepest point, will go underneath the Manukau Harbour, at a depth of around 110m. Along its path will be smaller pipes that intercept overflows as they enter creeks such as Meola.

It is, on its face, an elegant solution. The council estimates the many dozens of overflows into Meola Creek will drop to just a handful, and other streams in the city will have no overflows at all. When it is completed (expected in 2028), not only will the contamination of inner city beaches sharply decline, it will happen almost overnight.

LAWRENCE SMITH/STUFF Toilet paper and other rubbish attached to a tree long after a wastewater overflow.

There was an old saying about Auckland – the Waitematā was its gateway, and the Manukau was its back door.

Jim Jackson grew up with this mentality. More than half a century ago, he would take an empty sugar bag down to the harbour near his childhood home in Onehunga and collect scallops. It would take around 30 minutes to fill the bag, he recalls. Today, there are no scallops at all.

The Manukau Harbour, south of the city, is the second-largest in the country, and triple the size of the Waitematā.

It is shallow – about 6m deep, on average – and contains vast, intertidal mud and sand flats. It is a valuable habitat for migratory birds, which feast on the shellfish along the flats. A recent paper said the Manukau was the "single most important harbour for migratory wading bird species in New Zealand," noting that around one quarter of the country's wading birds are in the harbour at any one time.

The Manukau has many of the same water quality issues as the Waitematā, plus a few more.

Every few years, the council assigns its waterways a letter grade between A and F based on overall health. In 2018, the Manukau was awarded an E grade, a reduction from its D grade in 2016. The central Waitematā, where the overflows from Meola end up, scored a C.

The Manukau scored particularly low on ecosystem health and water quality. Many of the monitored sites on the harbour breached numerous water quality and nutrient thresholds; the overall water quality of the harbour "remains poor", the review concluded.

"These things didn't take a year, or five years, to find out," Jackson, who is chairman of the Manukau Harbour Restoration Society, says.

"They were 50 years in the making."

SUPPLIED The wastewater treatment plant at Mangere, which treats most of Auckland's wastewater.

The largest river in Auckland comes out of a tight collection of low-lying buildings near Mangere.

The Mangere wastewater treatment plant processes between 300m and 400m litres of water per day, which flows out of the plant at a rate faster than any natural river in Auckland. Wastewater is piped from around the city to the plant, where it undergoes a five step treatment process before it is discharged at high tide. It takes around 12 days for this waste to drift west out of the harbour, into the Tasman Sea.

The plant's owner, Watercare, says it is one of the 10 best facilities of its kind in the world; when it was built in 1960, its sprawling oxidation ponds - since removed - were the largest in the world.

During a large storm in 2016, the plant was processing around 12,000L of water per second, enough to fill an Olympic-sized swimming pool every four minutes. That was only two-thirds of its capacity, which it has never reached.

Despite the plant's impressive ability to treat waste to a high standard, some observers say it is holding the harbour back.

A major issue, they say, is that the treated wastewater is freshwater, while the harbour is saline. Because the freshwater from the plant is the biggest inflow into the harbour, it is likely changing its composition in a way damaging to some saltwater species. In a precarious ecosystem, even small changes can have an impact - if marine life is affected by the mixture of fresh and salt water, it could have a ripple effect up the food chain.

Another concern is what happens during heavy rain.

When demand is high, two of the plant's five treatment steps are bypassed, so water can be processed more quickly.

Watercare says this only happens around 0.8 per cent of the time, and the three step process is still full treatment (it calls its usual five step system "advanced treatment" - it says the three step process used during bypasses would be the default treatment level in other countries). But by not treating the water to the highest standard possible, there are concerns it may be adding nutrients to the already nutrient-rich harbour.

"What goes out during those [bypass] periods is a higher strength waste," says Gemma Tolich Allen, an independent wastewater biologist, who formerly worked at the plant and advises the Manukau Harbour Restoration Society.

"It is treated – it's not raw sewage in any sense – but it's not treated as fully as it would be on a dry day flow.

"Over the last 20 years, it has, to a great extent, negated the improvements we would have seen in the Manukau Harbour if all of the waste was treated inside the plant."

PETER MEECHAM/STUFF The entrance to the Manukau Harbour.

The Central Interceptor will be bringing up to two billion additional litres of overflow into the plant each year, which will be treated and end up in the harbour.

"[The interceptor] will protect the beaches on the Waitematā, very much so," Tolich Allen says. "It will be a huge improvement for them.

"Unfortunately, because of these high storm flows, they don't treat it fully and the level of nutrients going out on those occasions is higher than it should be. That will increase if the Central Interceptor is taking increased flows from the rest of Auckland."

By protecting beaches in the swanky suburbs of Herne Bay and Point Chevalier, is the city putting additional pressure on Manukau Harbour?

Watercare says it's not that simple. Although there will be additional waste going to the plant, Watercare's modelling suggests the load will increase less than 2 per cent, while it expects the number of bypass events to stay the same.

"When it comes to wet weather, we wouldn't see a difference because we can only put a maximum flow through the plant," says Watercare's network efficiency manager Anin Nama.

"What the Central Interceptor is designed to do is make sure we don't exceed the current throughput we do today."

He says one of the most important functions of the tunnel is storage. It will be able to hold around 200m litres of water, which can be slowly fed into the plant. When the tunnel overloads, there will be spills up the network, mostly where they already happen, around Meola Creek.

Those spills would be significantly less frequent, he says, happening at 10 locations, rather than the more than 200 they do today.

He adds that the interceptor is just one piece of the puzzle. Watercare will continue to separate the stormwater and wastewater networks in older parts of the city, as well as optimising its other treatment plants to spread the load.

"We're doing a toolbox of things - we're doing separation, we will be taking some flows to the treatment plant, we'll be looking at soakage as a method of separation," Nama says.

"The key thing we need to realise is there will be a substantial improvement in the environment as a result of the total works."

TOM CARNEGIE/STUFF Jim Jackson, of the Manukau Harbour Restoration Society, above a wastewater overflow at the eastern beach of Taumanu park.

Even if the Central Interceptor doesn't add problems to the Manukau harbour, the status quo is seen by some as shameful.

"We are the poor cousins of the Waitematā and the Kaipara," Jim Jackson says.

"The water stinks, it's warmer than the surrounding environment. If you look at the outlet at the [treatment plant] it looks like Huka Falls when they let the water go. No one in Auckland really realises that.

"I don't think this harbour can accept more and more treated wastewater going in and damaging some of these important ecological areas."

He says the city, and even the country as a whole, has prospered on the back of the harbour. As the city expanded, accommodating more and more people without commensurate attention to the services needed to cope with that many people, it has been up to the harbour to absorb that neglect.

The Central Interceptor taps into those decades-old tensions; Manukau as the city's exit, not its entrance.

It's a tension deeply rooted in colonialism, dating back to the Crown's confiscation of Māori land around the harbour beginning in the 1860s, little of which was given back. While the city developed, its most pollution-heavy activities were reserved for Manukau.

Among those activities was the treatment plant itself, which took over much of the harbour with its oxidation ponds. To build the plant, the Oruarangi River, as well as another stream, was blocked off and access to the harbour became difficult. Creeks were buried, and paua and scallop beds were destroyed.

At the same time, the international airport was being built on the harbour's southern end, and industrial polluters were being allowed to discharge into the harbour. At one point there were dozens of polluters, including three meat works, three fertiliser factories, two wool scours, an abattoir, and a steel mill, as well as leachate from numerous rubbish dumps and run-off from the council's septic tank, all flowing into the Manukau Harbour.

The decades-long degradation of the harbour was particularly damaging for the Makaurau marae at Ihumātao. The wastewater plant was built adjacent to the marae, desecrating a valuable seafood resource while bringing an influx of midges and the stench of human sewage.

This, and other issues, were central to a major Treaty of Waitangi claim concerning the Manukau Harbour. The claim said in exchange for agreement to the wastewater plant, Makaurau marae was offered quick connection to the sewerage system, access to the harbour, and compensation of $8000.

As the claim noted, none of this happened; the marae was among the last communities in the city to be connected to the sewerage scheme, a process that took 30 years. Access to the harbour remained limited, and compensation was never paid.

"The claimants do not want compensation," the Waitangi Tribunal concluded. "They want things restored to what they were. Regrettably, that is unrealistic."

CHRIS MCKEEN/STUFF A banner hanging on the old Mangere Bridge during a hikoi related to the Protect Ihumatao movement. The degradation of Manukau Harbour has caused longstanding tensions with the community.

The Manukau Harbour has been on an upward trajectory, albeit a slow one. The oxidation ponds were removed in the early 2000s, freeing up that part of the harbour, and many of the industrial polluters are gone. The Oruarangi River flows again, albeit as a shadow of its former self.

Regular monitoring since 1987 shows the harbour's water quality has steadily improved, even though it remains low.

The solution to the city's wastewater problem may need to be more drastic than simply treating more water; it may require a rethink of the city's relationship with water as a whole.

"Really, everyone should be using water sensitive design," says Liz Walker, from the Meola Creek group.

"Cities like Sydney, Melbourne… People there use water sensitive design, people see it, love it, and start to use it. None of that is really happening here."

While she'll be happy to see Meola Creek finally cleaned up after nearly a century of pollution, Walker says she's worried it will be at the expense of the Manukau, and the root problem isn't beng fixed.

"It's a low energy, saline environment, and you're suddenly pushing this fresh, clean, beautifully processed water into the Manukau Harbour," she says. "It's an issue".

For Jim Jackson, he hopes it won't be left to the next generation to restore the Manukau to its former glory.

"If we're not careful, in 50 years time, another generation may wake up and say, 'why didn't you do something?'"