One of Auckland's most popular beaches is off limits to swimmers for the next few days after tests found the water could harm people's health.
Warning signs have been put up at Mission Bay on Auckland's east coast warning people against swimming there.
The Auckland Council says levels of bacteria high enough to be a risk to human health were found during testing on Wednesday.
The council is not sure what the source of the bacteria is, saying officers have checked the wastewater and stormwater network and found no issue.
It says an animal carcass or waste dumping from a boat could be to blame, or the test results could be a false positive.
The council said nearby Kohimarama and St Heliers' beaches are safe to swim at.