There are concerns over the potential effects of an approved herbicide to kill the Nardoo weed in the Collie River is not being monitored.
Concerned river expert Ed Riley said a the Department of Water Environment and Regulation gave the Shire of Collie permission to undertake spraying on the Nardoo in the river.
"The chemical removal of Nardoo runs the risk of possible poising of non targeted species both flaura and fauna, without motioning it is not known what vertebrate, macroinvertebrate, nature, fish and river weed are coping," he said.
Shire of Collie David Blurton said there had been a long history of Nardoo in the Collie River, a green weed.
"There has been a long history of Nardoo choking the river and nobody did much about it so council took it upon themselves to address it," he said.
"We have gotten rid of most of it, but it grows back so it's an annual thing."
Mr Blurton said the shire undertook tests before the herbicide was initially sprayed onto the Nardoo and after it was sprayed to see if anything had been effected.
Mr Blurton said that it was an approved treatment.
"When we were initially deciding what the best treatment was for the Nardoo we were doing a lot of study around the impacts of that spray, plus some of the other things we tried," he said.
"Ultimately this was the best treatment and the most cost efficient."
They did monitor it as part of the experiential process but no longer monitor it now.
"We anecdotally look and check but we don't scientifically analyse everything. We don't have to monitor it," he said.
University of Western Australia Centre of Excellence in Natural Resource Management professor Peter Cook was previously a consultant for the Shire of Collie.
"When I was the consultant it was when they first started using the herbicide redlone, which is being used now," he said.
He said part of the agreement with the Department of Water Environment and Regulation was that the shire had to monitor what happened.
"Part of the deal was that you had to monitor what happened, in other words we need to know that the spraying works on the Nardoo but also that it's not killing anything else, and the only way you can do that is by properly monitoring it," Mr Cook said.
However, Mr Blurton said the agreement only needed monitoring in the first experimental process and not all the time.