An "extreme risk" health warning has been issued about potentially deadly toxins in a Waikato lake, but those affected may not notice symptoms immediately.
A cyanbacteria algal bloom. (File photo) Photo: 123RF
The warning has been issued for Lake Ngāroto, after a significant increase in cyanobacteria and E.coli levels.
People should not do any recreational activities at the lake as skin contact with the water, or swallowing the water, is dangerous, Waikato DHB said.
"The cyanobacteria currently in the lake produces toxins which are extremely harmful and may cause serious illnesses or death."
Symptoms from swimming in a bacteria filled lake may take some time to develop, it said.
DHB medical officer of health Dr Felicity Dumble said the bacteria produce irritating toxins.
"It can cause a rash on your skin, it can irritate your eyes... if you swallow it, it can also irritate your stomach or cause diarrhea or vomiting."
Dumble said in particular, the toxin could affect people with existing health conditions like hayfever and asthma.
Anyone who has had contact with the water and develops symptoms should contact their GP and alert the DHB.
Dogs are particularly sensitive to cyanobacteria, and owners should not let their animals drink or swim in the water.