The Cawthron Institute is defending its research which suggests the Manawatu River is heavily polluted and says it never stated the pollution was the worst in the world.
The research by the Nelson-based institute was released late last year and has been strongly criticised by some farmers for its lack of detail.
It showed the the Manawatu River has the highest reading of depleted dissolved oxygen of 300 rivers and streams in North America, Europe, Australia and New Zealand.
Federated Farmers has called for the Institute to name the other 299 rivers on the list to back up its research.
The institute's ecologist, Dr Roger Young, says the research was designed to give the Manawatu-Whanganui regional council an indication of the Manawatu river's health.
He says the indicator used was a relatively new one, which about 570 sites are now using, and the measurements from the Manawatu are the highest.
However, he says 570 sites represent a small number of sites worldwide, and he has never described the Manawatu River as the worst in the world.
Dr Young says the Manawatu River is unhealthy, as shown by other indicators of river health.
He says the institute will release a list of the random testing sites this week.