After the most recent testing last week, the south Piha lagoon was flagged as unsafe for swimming on Auckland's Safeswim website.
Last summer, it exceeded the safe level for E. coli bacteria indicators in 11 of 21 weekly tests.
A public health warning sign has been put up at the site urging people not to swim or collect shellfish. Yesterday, however, holidaymakers continued playing in the lagoon.
"The Piha Lagoon is next to the main parking and picnic area and people see it as safe for their children but are blind to the fact that the water quality may be poor, though the water quality on the beach itself is excellent," said Piha historian Sandra Coney, who chairs the Waitakere Local Board.
In the backdrop to the lagoon, which is fed by stormwater drains, homes rely on individual septic tanks with land soakage areas.
"It's been ascertained at Piha, and Karekare, that the problem is malfunctioning septic tanks," said Ms Coney. Residents are bringing pressure on the council to take action, because they don't like the lagoon being closed off - it's like a blot on the beach."
Ms Coney said the local board had only $50,000 from which to make small grants to encourage repair or replacement of tanks. The problem had intensified due to changing use of the small coastal community.
"There are old systems made for when people came to their baches over summer and did not have dishwashers or flush toilets.
"Suddenly, baches have expanded into houses for year-round living and the septic tanks can't cope with that."
Council land and water team leader Phil Brown said the council would commission the Ecomatters Waitakere Trust to give door-to-door education to Piha householders, and also offer a free assessment of tanks by an independent wastewater engineer.
The council issued six insanitary-building notices in the year to December 2013 - out of 670 septic tanks in the west coast lagoon area.
However, most complaints are about tanks built before the Building Act came into force - before 1993.
In these cases, the council must have evidence that raw sewage is surfacing from the tank soak field or that there is a constant smell.
Council environmental science manager Dr Martin Neale said microbiological tracking had achieved "mixed success".
Sewer exit gets safety tick
One of the safest places to swim in the Manukau Harbour is close to the outflow for Auckland's main waste water treatment plant, says a public health expert.
Dr Francesca Kelly said the plant's disinfection processes had shown improved performance since an upgrade 12 years ago.
The Watercare plant tested effluent discharge after it had received doses of UV irradiation for disinfection.
"This confirms to us that the prospect of pathogenic viruses coming from the plant is exceedingly low," said Dr Kelly, who chairs the independent microbiological quality review group.
Dr Kelly said the greatest risk to humans was when waste did not get to the plant. After storms or heavy rain, raw sewage and other sources of contamination entered the harbour and beaches from stormwater drains, and it became unsafe to collect shellfish.
The nearest Auckland Council test sites to the plant on the harbour's south side are Oruarangi Awa Creek and Mangere Bridge - declared safe for swimming on the Safeswim Auckland website.
Since 2000, a permanent health warning has been posted at Weymouth Beach. The council is investigating the pollution.
Monitoring sites last summer showed the safe level of pollution was exceeded at Wood Bay six times, Green Bay five times, Laingholm Beach four times and French Bay and Huia Beach three times.
The suspects
• Karekare Lagoon - human and dog faeces.
• Piha Lagoon - human and wildfowl faeces.
• North Piha Lagoon - old, or partially septic tank-treated human faecal sources.
• Bethells-Te Henga Lagoon - mainly livestock but also human, dog and wildlfowl faeces.
• Sometimes, sources of faecal pollution could not be identified in all lagoons.
Source: Auckland Council 2014 working report
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