It could be several years before some of Auckland's most frequently contaminated beaches can avoid the impacts of heavy rainfall.
More than 30 beaches across the city were deemed unsafe for swimming after this week's heavy rain, despite ongoing work to improve storm and waste water infrastructure.
One of them - Judges Bay - suffers from very old wastewater and storm water networks, Swimsafe's Nick Vigar said.
"You would expect Judges Bay to go red after something like five to 10mm of rainfall, which is fairly routine rainfall events, compared to some beaches which might take 10 times that amount."
Auckland had 100mm of rain this week, leaving popular spots like Mission Bay unsafe.
Auckland Counci's Swimsafe website shows water quality around the city, with red meaning the risk of illness from swimming is high.
"There're a range of reasons why they're going red, there's bird poo, dog poo gets washed off surfaces. As we get bigger rainfall events we start getting waste water overflows," Vigar said.
Black pins show areas with a very high risk of illness, after sewage overflows.
Hundreds of millions of dollars is being invested to fix the problems, but it will take years.
The biggest project is the central interceptor pipe, which will eventually take wastewater that would wash out to sea to the treatment plant in Māngere.
In the meantime, people are being encouraged to do their bit, to keep beaches safe to swim in this summer.
"The main thing to remember is that there's only three things that you should put down the toilet - pee, poo and paper," said Watercare's Brett Evans.
"Not wet wipes, not sanitary products."
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