Waikato Regional Council environmental monitoring manager Dr Ed Brown said it's not possible for the council to monitor all locations where people are in the water.
"So it's always safest that you check the water before you get in. Algal hotspots include small lakes and side arms of the hydro lakes where water can be stagnant and the water temperatures are higher than for flowing rivers.
"These factors can sometimes support the rapid growth of algae, resulting in very visible algal blooms, sometimes described as 'pea soup'," said Ed.
He said test results should be used for general guidance only.
"Conditions can change between tests, and cyanobacteria and toxins are not evenly spread through lakes, so may be concentrated in some areas by wind and water movements. That means even where monitoring does occur it is important to check the water before you get in.
"You should also avoid swimming in rivers, streams and harbours for 48 hours after heavy rainfall events. That's because the water is likely to be contaminated with animal faeces from rural and urban runoff, that may cause a tummy bug, sore throat or skin infection.
"If the water looks discoloured, smells unusual, or if there is scum on the surface of lakes or on the beds of rivers, swim or play somewhere else," he said.
If concerned, members of the public who see a possible bloom, are urged to phone Waikato Regional Council on 0800 800 401.
Meanwhile, anyone who has been swimming and is concerned about their health is urged to contact their GP or the Public Health Unit of the Waikato District Health Board on 0800 800 977.
For queries regarding Lake Taupō or the upper Waikato River from Taupō to Lake Maraetai, contact Toi Te Ora Public Health on 0800 221 555.
Updates on health warnings can be found online:
Waikato Regional Council:
www.waikatoregion.govt.nz/algal-blooms-in-the-waikato-region
LAWA:
www.lawa.org.nz/explore-data/swimming
