Cook Islanders have been told not to swim in the Titikaveka lagoon while water tests are being carried out.
Samples from the lagoon are expected to arrive in New Zealand today and will be tested for the microorganism dinoflagellate.
New tests are performed out after swimmers in the lagoon continued to experience skin and eye irritations.
A public notice has been issued asking residents, especially those suffering from asthma and other medical conditions, to refrain from swimming in the area.
A Ministry of Health spokesperson, Vaine Teokotai, says the plankton-like organism is present in the water and carried by wind to land.
He says the main problem occurs when people get out of the water because the toxin attaches itself to wet bodies.