Milford beach in Auckland has nearly 15 times the safe amount of faecal bacteria, and an outlet that drains into Castor Bay has more than 400 times the guideline, Safeswim says.
Water quality tests from Tuesday morning remain "below recommended guidelines for swimming at Milford", Safeswim Technical Lead Dr Martin Neale said.
Samples from Milford North and Milford Central were both in excess of 4000 enterococci, compared with a guideline value of 280, and the sample from the Wairau Outlet was in excess of 240,000, compared to a guideline of 550, he said.
The unsafe bacteria levels come after a major sewage leak on Sunday morning which evacuated homes.
Watercare said two properties on Alma Road needed interior commercial cleaning and a third and fourth property needed to be cleaned underneath their houses
Its wastewater transmission manager, Chris Harbour, said it was one of the worst wastewater spills he had seen in his 25 years in the role.
"I have certainly seen much bigger pipes than this one break but in terms of impact on residents, unfortunately this is one of the worst incidents I've dealt with."
Temporary repairs to the broken pipe have held and a permanent repair was being considered, Watercare said.
It said possible third party damage may be to blame.
"This was a strong concrete pressure pipe," said Mr Harbour.
"These pipes don't typically corrode because they are 100 percent full during normal operating conditions, with no access to air. Therefore that leads us to believe that the pipe has been damaged at some stage, creating a weakness, leading to the pipe blowing out in such a dramatic way."
Meanwhile, Safeswim said water quality alerts saying it was unsafe to swim at Milford Beach would remain in place on the website and warning signs would remain on the beach until testing proved the water quality met recommended guidelines.