Takapuna Beach on Auckland's North Shore was polluted by sewage at the weekend, but because of a bureaucratic bungle, the Safe Swim website assured the public it was low risk.
Beachgoers who spotted a "public health warning" sign that was at odds with the website are upset.
"Had our two-year-old son down there this afternoon - I'm just livid," one wrote on Facebook.
Wastewater utility Watercare Services, an Auckland Council-controlled organisation, said that on Saturday a large build-up of fat and rubbish in a sewer caused an overflow at a private property on Clifton Rd.
Workers had difficulty clearing the pipe so large suction trucks were used. Once the blockage was removed the workers cleaned up the surrounding area.
"Unfortunately, our team were unaware that wastewater from the overflow was entering the stormwater network and discharging on the beach.
"This was reported to us by Auckland Council's pollution control team who put up public health warning signs on the beach on Saturday evening, ahead of Sunday's beachgoers.
"Our crew responded on Saturday evening by cleaning up the residual wastewater.
"An overflow on to a beach in dry weather is usually signalled with a black flag on the Safe Swim website as well as pollution control's on-site signage.
"Unfortunately, on this occasion, the internal escalation process was not followed correctly and the Safe Swim team were not alerted to the overflow until early Sunday evening.
"By then, there had been several tidal cycles and the beach was deemed safe to swim at."