The extensive wet weather in Auckland this year has caused more stormwater overflows than the past two years combined.

Auckland has had two years' worth of sewage overflow into its natural environment after the worst rainfall in a decade.

A pungent mix of raw sewage and stormwater from areas of the city served by a century-old sewerage system is pushed out to sea when it overflows during periods of heavy rain.

In the year to July 2017, a period including Cyclone Debbie and the Tasman Tempest, there were more than 373 overflows – more than the previous two years combined.

WATERCARE Auckland's combined sewer system overflows into creeks and the ocean when it is overwhelmed by wet weather.

The Auckland Regional Public Health Service is now investigating "moderate" rises in some types of gastroenteritis, and their links to the overflows.

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PUBLIC HEALTH CONCERNS

WATERCARE Areas in pink are served by a combined sewerage network and represent two per cent of Auckland's wastewater network.

Mayor Phil Goff said concerns about the overflows had prompted him to launch real-time water monitoring of Auckland's beaches.

The overflows of sewage-infused stormwater into the Manukau and Waitemata Harbours are a legacy of Auckland's combined sewer system.

Developed in the 1900s, it sends stormwater and sewerage down the same pipe in a patchwork of older Auckland suburbs from Blockhouse Bay through to Three Kings and Remuera.

SIMON SMITH/STUFF John Seward had a sewer overflow onto his private Te Atatu property in April.

This wastewater is "diluted" by stormwater, Watercare network manage Anin Nama said.

"Because it's heavily diluted, the environment can assimilate some of that diluted wastewater over time.

"I've got more sympathy for those people that have water courses going through their property and that's what we're trying to address."

CATRIN OWEN/ FAIRFAX NZ Charlotte Fisher from Westmere with rubbish collected from combined spills at Cox's Bay and Okahu Bay, including tampons and syringes.

Wastewater biologist Gemma Allen Tolich said the overflows increase, rather than dilute, the concentration of sewage in beaches and streams.

E.coli samples taken during an overflow in 2010 at Meola Creek detected the sorts of figures that would be found in raw sewage entering provincial wastewater treatment plants, Tolich said.

SEPARATING THE PIPES

Only 16,000 households, or two per cent of Auckland's wastewater network, use a combined sewer system.

Responsibility for separating the system and installing new stormwater pipes rests with Auckland Council.

By 2021, 688 properties will have their stormwater and wastewater separated, Watercare stakeholder liaison Brent Evans said.

Watercare's $926m Central interceptor, a huge pipe that runs from Western Springs right through the city, will also transport 80 per cent of the unseparated sewage and stormwater to its Mangere Treatment Plant when it is completed in 2026.

Evans said overflows on private property were a bigger issue from a public safety point of view and that was what happened on parts of the network that weren't separated.

"If separation was the answer we would do nothing else but separate all the properties."

Ian Seward found his house flooded with tampons and sewage in April after a manhole overflowed.

Watercare said that was caused by a number of people who had illegally connected their stormwater to their wastewater.

Each household that did that displaced the wastewater of 40 others, Evans said.

"Now if Auckland Council says 'we're not going to invest adequately in stormwater', what that means is we're left trying to deal with it."

WORST RAINFALL IN A DECADE

Auckland had an entire year's worth of rainfall in the year to September, Metservice meteorologist Georgina Griffith said.

"We've had wetter years but this year's probably been the worst we've had for a decade or so."

Evans said with $2.8bn of infrastructure works in the pipeline for the next 10 years, it would be wasteful to design for "outlier" years like 2017.

"We'd be investing way before we actually needed to, just to deal with extreme events.

"We want to take the stormwater out of the wastewater network, not encourage bigger pipes to accommodate it."