Health and council officials are monitoring for blue-green algae in Bay of Plenty lakes and rivers.
The Bay of Plenty Regional Council and Toi Te Ora - Public Health Service are taking weekly samples of the micro-organism which causes the bloom.
If significant blooms are found health warnings are issued.
Medical Officer of Health, Phil Shoemack, says over the last couple of decades the toxic blooms have been detected in rivers and lakes.
He said they can trigger asthma and allergies, cause rashes and nose, ear and throat infections.
Dr Shoemack says there are things people can look out for.
"Look out for discolouration of the water, which could be from light brown right through to bright green and a smell which is sometimes, present but not always.
"If you see water that looks other than what you expect it to be, if it looks unusual, smells unusual it could be affected by algae and you're better to go elsewhere."
The council will erect warning signs if the algae is detected.
Monitoring started on 15 December and will continue until the end of summer.