No summer swimming at several Auckland beaches

Stormwater drain (Getty)
It only takes 5mm of rain to fall before Auckland's combined sewage and stormwater pipes overflow (Getty)

The equivalent of 400 Olympic swimming pools of sewage is seeping into Auckland's harbour every year.

Auckland's century-old stormwater pipes overflow into the ocean almost every time it rains, and it's putting many of the city's beaches out of action.

It only takes 5mm of rain to fall before the combined sewage and stormwater pipes overflow.

"The frequency with which it happens in Auckland is a bit of a concern, because it's 50 times a year," says Auckland University wastewater engineer Dr Lokesh Padhye.

A Weekend Herald investigation found in a year, around 1 million cubic metres of wastewater is disposed of in the harbour each year.

Hauraki Gulf Conservation Trust chairperson Ian Burrows says it's alarming, considering the already less-than-ideal state of the harbour.

"Anything that's putting waste or contamination into the Hauraki Gulf is a concern from the trust's point of view."

Ten beaches are now deemed too polluted to swim in, with some so bad Auckland Council are giving up on taking measurements there.

New housing developments the city desperately needs are expected to make the problem worse.

"[Auckland Council] are pushing ahead with developments well in excess of our carrying capacity," councillor Mike Lee told the Herald.

Most developed cities have moved to separate their stormwater and sewage pipes in recent decades, but Auckland lags behind - particularly on the central isthmus.

Dr Padhye says city 'greening', planting on roofs and walls, is one way Auckland can mitigate its stormwater issues.

Mr Burrows says it's not just an environmental issue, but an economic one.

"We've got a family here for example who a few years ago set up a snorkelling trail - you just can't see nowadays because the sediment that's running into the bay."