A council spokeswoman said yesterday that during a process of elimination in the pipe network, a significant leak into one pipe was found. It was being repaired by Watercare Services.
The council stormwater team was now testing a second pipe.
However, because it served a bigger catchment than the first, it might take longer to find the problem.
Council sampling over three months of last winter at Onehunga picked up "very strong" contamination by dog and human faeces bacteria.
Colin Tunnicliffe, an Onehunga resident who had criticised a lack of provision in the park plan for improving water quality, said he supported the council's clean-up of stormwater.
However, he said raw sewage would still be a problem because Watercare had no plans to shift its wet weather overflow pipe.
A Watercare Services spokesman said the outflow was for a pumping station 12km away and had overflowed 27 times between January 1994 and April 2012.
Onehunga Enhancement Society chairman Jim Jackson said the council would have done nothing if the foreshore restoration project had not brought pollution to public attention.
"The restoration project can be completed but during storm events and raw discharges into the Manukau Harbour, warning signs will have to be erected similar to the beaches on Tamaki Drive.
"Another solution would be to have Watercare construct an enclosed storage tank as part of the project to contain the material then return it into their system over the following 48 hours for normal processing at the Mangere Treatment Station."
