The black mark at Okahu Bay, Auckland.
It offers stunning views of Auckland Harbour, its foreshore is great for sun worshippers but swimming may be unwise at Ōkahu Bay.
The popular beach was last week stamped with a new "very high risk of illness" water safety warning from Auckland Council - but it turns out the precise "risk" is hard to quantify.
In a bad week for swimming across Auckland, with up to 48 beaches classified as unsuitable for swimming, Ōkahu Bay stood out as the only beach with the black logo - the highest warning - on the council's Safeswim website.
The new "very high risk" warning was implemented at the start of last month and indicates a direct contamination from a human wastewater system.
Ōkahu Bay, just east of the CBD, was labelled with it from Monday, January 14, to Thursday, January 17.
Auckland Council Safeswim programme manager Nick Vigar said the red classification of "high risk of illness", below the new black rating, denotes a greater than 2 per cent chance of getting sick from swimming at a beach.
The red classification is triggered by a direct measure of faecal indicator bacteria, Enterococci, in the water.
However, the percentage chance of getting sick on the black "very high risk of illness" warning was indeterminate.
Rather, it indicated it was directly contaminated by human faeces, and not just the Enterococci bacteria that could be from the gut of any animal.
"Among public health officials there is a general acceptance that if that bacteria comes from a human source then the risk is much higher because all those pathogens in there are human specific," Vigar said.
"The trick about that is it's very difficult to estimate exactly how much more the risk is. We can't put a number on it and say it's 5 per cent or 10 per cent because it varies and that's what the very high risk is about, the black symbol.
"This might be due to heavy rainfall or it might be due to a fault in the network, a blockage. It'll stay black for 48 hours, to give the wastewater time to clear from the system."
Vigar said the most recent alerts across dozens of beaches last week were from rainfall over 48 hours from last Sunday.
Ōkahu Bay was particularly problematic because the area had partially separated and partially combined stormwater and wastewater systems.
"As such it suffers from more frequent overflows than a separated wastewater catchment," Vigar said.
"However, presently the Safeswim website treats it as separated, which is why you see the very high risk black alerts here, where a fully combined area, such as Herne Bay or St Marys Bay, would display a high risk red symbol.
"In fact, Ōkahu Bay has far fewer overflows, and better water quality, than the fully combined areas."
Despite the potential health risks, a one-hour survey of the beach from 3pm to 4pm last Tuesday had 17 people swimming on an overcast day.
There was no on-site signage of the "very high risk of illness" warning at Ōkahu Bay.
Papatoetoe resident John McKillop was surprised but not deterred from swimming after being informed of the warning.
"I suppose it does bother me. If there were signs up, but there's not. I assume it's all right if there're no signs up," McKillop said.
Christchurch resident Rory Jones was heading out from Ōkahu Bay for a day of sailing with his son. He wasn't aware of the health risk either.
"You have these beautiful natural assets that you suddenly can't use," Jones said.
"But if people don't show up and use them there's no imperative to improve them."
Safeswim programme improvement manager Anin Nama said a $15 million construction project was set to start in October to fully separate the stormwater system from the sewerage system at Ōkahu Bay.
"This black flag thing is only in its infancy over the last couple of months, so there will be some teething issues that we need to clean up in the coming months," Nama said.