The Manukau Harbour Restoration Society is calling for more transparency on the amount of partially-treated wastewater that is pumped into the harbour.
Watercare's Mark Bourne said a $500 million investment made in the plant in the early 2000s significantly improved the health of the Manukau Harbour.
Last year, wastewater that did not have advanced treatment was discharged into the harbour 19 times.
Watercare said even during extreme storm events, which increased water flow through the Māngere Wastewater Plant, all wastewater got at least three stages of treatment.
Environment and Consents Manager Mark Bourne said there were five stages of treatment. By international standards three stages was considered full treatment.
"Around the world, most plants only provide up to three stages of treatment. This includes many of the largest plants in Australia, including the large plants in Sydney.
"Because Māngere Wastewater Treatment Plant offers very advanced treatment, it provides a further two stages of processing during normal dry and wet conditions," Mr Bourne said.
The Manukau Harbour Restoration Society said the semi-treated discharges were not good enough. The Māngere treatment plant was not keeping up with intensification.
Society wastewater biologist Gemma Allen said in one discharge in June 2014, the equivalent of 148 Olympic sized swimming pools of wastewater that did not get the advanced treatment was put into the harbour.
She said Watercare did not own the harbour, but was responsible for contributing to its ailing health.
"During dry conditions the treatment plant works very well, but when there is a storm or heavy rain we are seeing more of these discharges into the harbour.
"Watercare's own estimates are that these discharges are happening between 17 and 20 times a year and they've indicated that's going to increase to 22 events a year," Mrs Allen said.
She said wastewater that had only gone through three stages was pumped into the harbour from four to 32 hours at a time.
"Watercare and the Auckland Council are not prepared to talk in an open matter, they're failing to provide information."
Ms Allen said Watercare did not own Manukau Harbour, but, in a large sense, controlled its health.
"It's summer time and people are going to their local beaches to find health warnings and signs saying they can't swim - something needs to be done."
Watercare's Mark Bourne said: "It's easy for people to say we are responsible for polluting the Manukau Harbour, but that fails to recognise the impact of the thousands of other activities taking place within the catchment."