Watercare has explained why Aucklanders have been warned of a "very high risk" of swimming at popular Mission Bay beach.
The Safeswim site has had a black pin in place for Mission Bay beach since Saturday evening.
The black pin means there is a very high public health risk for swimmers due to high levels of faecal contamination - exceeding national guidelines for swimming.
Watercare told Newshub that the black pin was put in place after Watercare detected an overflow of wastewater from their network. It expects the issue to be resolved by Tuesday night.
"A black water quality pin has been raised at Mission Bay beach as a precautionary measure after we found parts of the local wastewater network had been blocked by concrete, from third party construction damage," Watercare head of service delivery Sharon Danks told Newshub.
Danks said that once the issue was identified, they worked quickly to resolve the issue but warned there is a "small risk" some wastewater may have made its way into Auckland Harbour.
"When the problem was identified, we worked with Auckland Council's pollution response team to sandbag the affected stream to contain the wastewater and pump it back into the wastewater network via a nearby manhole," she said.
"However, there was a small risk that some wastewater may have made it into the harbour at Mission Bay, which is why we raised the black water quality pin on Safeswim."
Danks said a temporary pipe to carry the wastewater has been built and will be used until Watercare can install new permanent pipes.
SafeSwim has also issued a new warning to Auckland beachgoers to "exercise caution" after stinging bluebottle jellyfish were spotted in the water.
A day after a hazard alert was in place for Piha Beach on Auckland's west coast, the alert is now in place for Muriwai Beach.
Bluebottles are bright-blue creatures that float on the surface of the ocean. They are technically siphonophores - colony organisms, not jellyfish - despite their appearance.
Their venomous tentacles cause an extremely painful sting which raises red welts. These can last several days.
According to the Ministry of Heath, the best care for a sting is to flush the stung area with seawater then immerse it in heated tap water.
Serious allergic reactions are rare, however an ambulance should be called for symptoms including shortness of breath, severe dizziness or signs of shock.