The Cook Islands Environment Service says high levels of phosphate and nitrates are continuing to be found in Muri Lagoon.
Information officer, Tu Tangi, says they are conducting periodic testing of the lagoon and surrounding streams and a campaign has been launched to stop the sources of pollution.
He says people have complied by moving animals away from streams and working on faulty septic tanks.
Mr Tangi says they will now tackle the next problem.
"The high levels of phosphate and nitrate do come from excess use of things like detergents or other chemicals that would be used for agricultural purposes. That's another area that we haven't yet touched on so obviously, with the high levels still coming up in that area, we now need to turn our attention on to the growers and to see if we can work something out with them"
Mr Tangi says some tourist developments are now using organic detergents which break down effluents.
And, he says a consultant has indicated that random samples of water show the problem is not as bad as first thought.