Elevated E. coli levels have closed off a popular swimming spot in the northern Auckland region, just ahead of summer.
Locals first raised the alarm when the Warkworth quarry lake, also known as cement works, turned bright orange - the result of an "algal bloom" in the water, Auckland Council says.
Auckland Council took a water sample and then also discovered "elevated E. coli levels".
"The presence of the algal bloom and E. coli could be related," Food Safety and Health Enforcement team leader Alan Ahmu told Newshub.
"For example, if pollutants containing faecal matter are entering the lake and providing a food source for the algae to proliferate."
"The presence of E. coli equally may have nothing to do with the bloom and could be from another source."
The council is working to determine what risk the water poses to swimmers - but erected warning signs around the water's edge as a precaution earlier this month.
It is not known when the water will return to its normal colour, or how long the lake will be considered unsafe.
The site is a former concrete factory which was abandoned in 1928. It is now a popular spot during the warmer months.