A dramatic animation, dubbed the poo tracker, has emerged showing raw sewage sloshing along Auckland's beaches for days after rain.
The animation, used by the Auckland Council to predict water safety and quality for its SafeSwim website, showed what happened after a typical winter downpour.
But overflows happen to a lesser degree, about 100 times a year, in some places in central Auckland.
The animation shows plumes suddenly appearing at several points along the coast - the outlets from the streams where raw sewage overflows after heavy rain.
* Animation from DHI Water and Environment courtesy of Auckland Council.
The plumes represent water contaminated with bacteria - including e coli and cryptosporidium - with red being the most intensely polluted flows.
The bug-laden plumes spend the next three days sloshing back and forth for kilometres around the harbour until the bacteria die.
Auckland Council's water manager Andrew Chin said its not just the sewage-filled water that's a problem when there's a big overflow.
"With that wastewater will also come all of the gross pollutants - the litter, the dental pics and tampons, the quite unmentionables that flow in. That stuff not only doesn't break down in the sea, it can get dispersed quite widely."
And Auckland's water quality problem went much further than the Waitematā Harbour.
"Piha, Bethells, Waiheke with the septic tanks ... there's sedimentation in the Kaipara, nitrogen down in Franklin district," he said
The two major projects to help fix the Waitematā overflows - the Central Interceptor tunnels and the Western Isthmus - would hugely improve the Waitematā problem, he said.
It could mean there were as few as two overflows a year, he said.
However, some environmental groups say the projects would not do enough.
Mr Chin said if Aucklanders really wanted to fix the problem they should get behind a targeted water rate that would be voted on this year.
Much more polluted waterways, like London's Thames, have been able to turn their water quality around, he said.