The Bay of Plenty District Health Board is warning people to watch out for hazardous algae on the region's lakes and rivers.
The board's Medical Officer of Health, Dr Phil Shoemack, says algae has been a summer phenomenon in the Rotorua area for 20 years, but the recent spell of warm, humid weather has made it worse.
Dr Shoemack says people need to look out for the blue-green algae, which can also form black or brownish slime-like mats that cling to rocks and logs.
He says too much contact with it could trigger ashma attacks or cause skin rashes, stomach upsets, headaches and even visual problems.
