Wastewater from a Wellington apartment block may have been flowing into a popular swimming spot for more than 10 years.
Although it's the middle of summer, the spot remains closed as sewage seeps into the harbour.
Wellington Water has traced the source of the contamination to a wastewater pipe, incorrectly connected to a stormwater pipe under a central city apartment building.
"Cross connections aren't uncommon - we find at least half a dozen of them a year, easily," Wellington Water chief stormwater adviser Ben Fountain said.
The apartment building was constructed in 2009, but the pipe problem may precede that. A solution is currently in the works, which should stop waste leaking into the harbour.
Last month, a major wastewater tunnel collapsed, sending the water into Wellington Harbour. That leak has been fixed, but contamination at the diving platform has persisted.
"Culturally, socially it's not acceptable to put sewage out into bathing spots, so no, we don't like it," Fountain said.
At this time of the year, there would usually be hordes of people lining up to do bombs off the swimming spot's diving platform - but on Wednesday, only waste is swimming in the water.
"It's pretty disgusting. I come down here pretty much every week, now I probably won't come down here again," one person told Newshub.
"It's just a poor effort from the council, really. There's families around here, kids swimming in here all the time," another said.
Now the source of the contamination has been identified, the platform should be open for the rest of the summer.