Nearly all Auckland's rivers and streams suffer "widespread" faecal contamination.
Auckland Council's latest river water quality report, which analysed data from 2016-2018 across 36 freshwater sites across the region, found the majority of urban and rural sites failing the Government's national freshwater standards for E. coli bacteria, a key measure of human and animal faeces.
Rivers in urban catchments had the poorest scores, due to a range of contaminants, but particularly E. coli - largely due to wastewater system overflows.
Rivers in rural catchments typically ranged from "poor" to "fair", and, as expected, rivers dominated by native forested catchments had "good" to "excellent" river water quality.
These findings were consistent with a major government report Our Freshwater 2020 , which found in urban settings 99 per cent of rivers exceeded one or more environmental guideline, although this covered just 1 per cent of the country's river length.
This was compared with pastoral and plantation forest catchments, which cover about half and 6 per cent of total river length respectively, where one or more guideline was exceeded in 95 per cent of waterways.
While the report did not focus on trend analysis, the water quality index still indicated a slight decline in regional water quality over the past three years of reports, looking at data back to 2014, with a slight increase in the number of "poor" sites and a decrease in the number of "excellent" sites.
Out of 11 urban streams, four were classed as "poor" and the other seven "marginal" when it came to the water quality index, a measure of ecosystem health.
One of those classed as "marginal" was Oakley Creek/Te Auaunga, which had seen a slight dip in health, despite years of intensive remedial work.
Friends of Oakley Creek/Te Auaunga chairwoman Wendy John said a lot of their work focused on riparian planting, which helped filter contaminants flowing over land.
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They had no control of waste and stormwater overflows however, which were the "major issue", she said.
In older parts of central Auckland, wastewater and stormwater flow into a combined network of pipes. When it rains, stormwater overwhelms the pipes, which are designed to overflow into waterways, including Oakley Creek.
Watercare's $1.2 billion central interceptor, scheduled to be completed by 2025 , was designed to address the majority of these overflows, along with the western isthmus programme of sewer and stormwater separation, due to be finished in 2028.
The giant 13km wastewater tunnel will be built from Western Springs, near Auckland Zoo, to the Māngere wastewater treatment plant.
When the interceptor and a 2km extension tunnel from Western Springs to Grey Lynn is built, the number of overflows at several central city beaches and waterways will be reduced by more than 80 per cent.
Herne Bay, Sentinel and Home Bay beaches will go from more than 52 overflows a year to 10 or less. It will be the same at a bad overflow at Lyons Ave in St Lukes which sends sewage past a nearby apartment building, along a walkway at the back of Mt Albert Grammar and into Meola Creek.
The interceptor will reduce the number of overflow points from 219 to just 10 and each of these is expected to overflow less than 10 times a year.
"You can plant a million trees, but until those projects are finished we can't do anything about the overflows entering the creek," John said.
While water quality issues remained, John said their planting work was proving a boon for biodiversity, and they hoped to encourage urban dwellers to value their waterways more.
"It really is a biodiversity hotspot. The new plants are increasing the habitat so we are seeing a lot more birds, and insects, and trees and plants hanging over the creek providing a better environment for native fish - it is a whole ecosystem."
Andrew Chin, of Auckland Council's Healthy Waters team, said the latest state of the environment report did not show anything they did not already know.
"But what I would say is two years ago the council sought a water quality targeted rate, so the impacts of that investment will not be seen yet."
The big investments in the central interceptor and western isthmus were "very significant" projects to turn water quality around, he said.
The central interceptor would collect wastewater from several link sewers and shafts along the route, including the Grey Lynn tunnel.
The Grey Lynn tunnel is part of a 10-year project to improve water quality in the western isthmus, which includes sewer and stormwater separation in Herne Bay, St Marys Bay, Waterview and pockets of Freemans Bay.
Turning around water quality trends was a "long-term game" and could take decades, Chin said.
They looked to overseas projects, such as that in Chesapeake Bay on the United States' east coast, which started in the 1980s and took about 20 years before it saw any improvements.
But there had been a momentum shift in the past several years, Chin said, with the city's SafeSwim monitoring programme playing a major part.
"When people started seeing real-time forecasting about the city beaches it brought water quality to their attention."
Regulations around building and development work regarding stormwater had also tightened up.
"If you think back to the 1980s there was no sediment control, and now it is standard practice. Water sensitive urban design is also a major part of a lot of new greenfield developments.
"It is never static and we are constantly looking at those rules, but it is also important they are still cost-effective and reasonable."