The Auckland Council says it has no firm plans to clean up a major groundwater pollution problem it has known about for 28 years.
An RNZ Insight investigation has discovered evidence of why the contaminated Onehunga Aquifer is rated as the country's fifth worst toxic site.
Heavy metal pollution is leaching from a former fertiliser site in Onehunga near Mount Smart Stadium, creating a plume of contamination in the groundwater underneath.
The polluted water discharges into the Miami Stream, turning shellfish green along Manukau Harbour.
Notes by a former Auckland Council employee show the then-council was first alerted in 1988.
The council launched a study it called the Green Stream project, which reported back in 2010.
The report found serious pollution of the groundwater, but found there was a low risk to Auckland's drinking water, which is taken outside the contaminated plume area.
The Auckland Council said it was managing the health risks and monitoring the site, but had no plans to decontaminate it.
It was investigating the possibility of improving the water discharged to the harbour as part of a major transport project, the East West Link, but that would depend on the New Zealand Transport Agency, it said.
The government said it was frustrated with Auckland Council, but could not force it to act.
Few people on the streets of Onehunga spoken to by RNZ had heard of the contamination problem.