Manukau Harbour's ailing water quality has gone from bad to worse with the latest Auckland Council data showing some areas over 10 times desired nutrient levels.
Environmentalists and nearby residents say the latest report is another indication the harbour has "borne the brunt" of Auckland's wastewater too long, and more needs to be done to turn the situation around.
The annual monitoring report comes just after the Manukau Harbour Forum received a scathing review, finding the body set up in 2010 in response to the harbour's worsening conditions was ill-equipped for the job, mainly due to being underfunded and inadequate resourcing.
It also follows a council environmental scorecard that showed ecology and water quality in the harbour had gone from a D in 2016 to E in 2018.
The Coastal and Estuarine Water Quality: 2018 Annual Data Report analysed water quality at 31 sites across the region's three main harbours: Kaipara, Waitematā and Manukau.
Using the water quality index, measuring a range of key indicators, more than half of the sites assessed had water quality that was "fair" to "good", the best being open coastal east coast sites and the outer Waitematā and Kaipara harbours.
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But in the Manukau Harbour five of the eight sites were classed as "poor", with the worst recorded at Māngere Bridge and Waiuku Town Basin. The best sites, Grahams Beach and Manukau Heads, where classed as fair.
Wastewater biologist Gemma Tolich Allen said it was an "ongoing source of disappointment" to see the latest results showing the Manukau Harbour as recording the worst water quality in the Auckland region.
Health grades within Manukau Harbour varied greatly, with sites in the wider harbour where there was more tidal flushing faring much better than others further in, such as Māngere Inlet and Pahurehure.
Parameters such as chlorophyll were seen to be over 10 times council guidelines in some of those inner areas, and phosphorus and nitrogen all showed a high level of exceedance in the northeastern part of the Manukau, closest to the Māngere Wastewater Treatment Plant.
Tolich Allen said under the treatment plant's consent partially-treated wastewater could be discharged during peak storm flows, which happened on average 18-22 times per year.
"These in turn fuel enrichment resulting in the poor water quality. With increasing population growth and increasing numbers of storm events per year as a result of changing weather patterns, this situation is unlikely to improve.
"The Manukau has borne the brunt of Auckland's waste disposal unfairly for too long, land development and stormwater overflows have been insufficiently managed to protect the harbour, and this has resulted in it having the poorest water quality in the region."
Watercare's Central Interceptor project, and initiatives fast-tracked through Mayor Phil Goff's targeted water quality rate, would help reducing overflows, she said, but more needed to be done.
"The situation will only deteriorate over time unless there is political will at a national level to begin the necessary planning and actions to begin restoration."
Manukau Harbour Restoration Society's Bronwen Turner said the poor water quality was impacting on the rare ecosystems found there, and recreation.
"It is a stunning harbour, and people want to use it," said Turner, whose family had lived on the harbour for more than 130 years.
"With more people living in Auckland, that is only going to increase."
Forest & Bird's Auckland regional manager, Nick Beveridge, who lives near the harbour in Titirangi, said the ongoing water quality issues were "disappointing" to see for marine life.
"Those high levels of nutrients will have an impact. They will affect shellfish stocks, that will in turn affect wading birds that feed on them, and so on.
"There are a lot of issues though, most of the harbour being shallow, estuarine, and with just the narrow heads for the water to flush out of. But it is clear more needs to be done to turn the situation around."
The report noted water quality in 2018 was consistent with the last trend analysis for 2007-2016. The next trend analysis would be completed this year, along with a five-year monitoring report of the harbour.
Auckland Council head of healthy waters strategy Andrew Chin said poor water quality in the Manukau Harbour was a historic issue, contributed to by sediment, nutrients, heavy metals, pathogens from sewerage and animals, and litter.
The council and Watercare had various wetland projects and infrastructure upgrades under way, including the Awakeri Wetlands Project, designed to reduce impacts on the harbour.
The council and Watercare were also working on modelling the relationship between land use, pollutants, rain events and rainwater flow to develop new methods to improve the condition of the harbour.
A Watercare spokeswoman said they had no comment on the council's report, but added the Māngere Wastewater Treatment Plant was fully compliant with its consent conditions, and that untreated wastewater was not discharged to the harbour.
In November 2018 four beaches in the Manukau Harbour had long-term public health warnings lifted - one of which, at Weymouth Beach near Manurewa, had been in place for almost 20 years.