Auckland Council is allowing developments to occur knowing there is no adequate stormwater system and this will result in more frequent harbour spills, says Watercare chief executive Raveen Jaduram.

The wastewater system is designed for toilet and other household wastewater and, as much as practical, stormwater needs to be removed, Jaduram said in an email to councillor Mike Lee last year

The email highlights different views between council and its water business about how to deal with overflows from 41 sites into the Waitemata Harbour almost every time its rains. About 99 per cent of the overflows is stormwater. Just 0.2 per cent, or two litres for every 1000 litres, comes from toilets.

Council's Healthy Waters general manager Craig McIlroy has told the Herald wholesale separation of 16,000 homes on the old combined sewer-wastewater does not make economic or social sense.

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It would cost $1.5b, on top of the $1.7b already planned for new interceptors and feeder services to the Mangere waste treatment plant, to build separate stormwater pipes for all 16,000 homes in the combined sewer-stormwater pipes, McIlroy said.

In the email, Jaduram was critical of council's approach to stormwater.

He said the inflow of stormwater into the combined system caused an immediate increase in flow that was significantly larger than the wastewater flow.

The use of detention tanks in new developments to slow down the stormwater flow when it rains "is not a sustainable solution", Jaduram said.

"What is required is investment by council in proper stormwater infrastructure.

"Continuing to accept stormwater into the wastewater system erodes hydraulic capacity that could be used to service growth and provide better levels of services to existing customers.

"Council is allowing developments to occur knowing full well that there is no adequate stormwater system and that additional stormwater discharges to a Watercare combined sewerage system is going to result in more frequent spillages," Jaduram said.

The Watercare boss stands by his comments to Lee regarding the use of stormwater detention tanks, but declined to be interviewed on the issues he raised.

In a statement, Watercare said a joint party made up of Watercare and Auckland Council staff is investigating wet-weather overflows and associated issues and will report back to council.

The findings will be part of council's next long-term budget that will go out for public consultation next year.

Last night, McIlroy said future growth will not have a significant impact on water quality issues.

"The adverse effect of more housing will be offset by short-term improvements such as requiring onsite detention of stormwater in combined areas before the new interceptors are built," he said.

Lee said Jaduram's email was revealing, honest and courageous for a public servant.

"It's a clear warning to Aucklanders that we have a major problem that has yet to be solved.

"Clearly there is a significant shortfall in parts of the city to the most basic facilities of any civilised society. What we are doing is dumping enormous amounts of raw sewage into Waitemata Harbour," Lee said.

In the meantime about 690 properties in Freemans Bay, Waterview, Okahu Bay and Newmarket are having their sewerage and stormwater separated.