About half a dozen people may have contracted gastroenteritis after swimming at contaminated swimming holes in the Bay.
Bay of Plenty Regional Council and Toi Te Ora have issued warnings about waterways and swimming holes which have recorded unsafe levels of bacteria.
Welcome Bay's Kaiate Falls recorded unsafe levels of faecal contamination three weeks ago and it was registered as a site of "caution" this week.
Chief medical officer of health Jim Miller said Toi Te Ora was aware of about six people who suffered vomiting and diarrhoea in the past month after spending time in the falls and other waterways.
While each person had spent time in or around potentially contaminated water, it was not certain the waterways were the definitive cause of their illness.
"Most people who get sick don't go to the doctor. We tend to see a little tip of the iceberg but individual cases, it can be quite difficult to put it down to one thing because people can pick up gastroenteritis from food that hasn't been cooked well, or contact with animals, as well as recreational water use."
The regional council last week issued a warning for people to avoid some waterways because increased bacteria could make them sick.
Regional council water scientist Paul Scholes said stormwater drains and ponds had had little or no flow, resulting in increased bacteria and a build-up of other contaminants which could be a health risk to people and pets.
The conditions could also affect the water quality in popular swimming spots, he said.
Last week, the Bay of Plenty Times reported a popular swimming hole at McLaren Falls and another further down the Wairoa River at Bethlehem were listed among some of the poorest in the country for recreational water quality.
The sites were included in nationwide data that showed about two-thirds of more than 160 monitored river swimming spots had ratings of poor or very poor last summer.
Of 28 sites in the Bay of Plenty, one - the Waimapu River at the Greerton Park Footbridge - had a very poor rating, while 12 others had a poor rating.
The warnings come as Toi Te Ora continue to monitor the Bay's shoreline for paralytic shellfish poisoning. The high level of the PSP toxin prompted the public health agency to issue a warning against consumption of shellfish from local waters which is still in place today.
People can view the region's water quality at www.boprc.govt.nz.