The Government and electricity giant Mighty River Power want restrictions lifted on banned pesticides they want to discharge into waterways - including the Waikato River - to control pest weeds.
But iwi groups, environmentalists, beekeepers and the eel industry are opposing the move, saying there is not enough evidence to show the chemicals can be released safely into the environment.
A hearing before the Environmental Protection Agency is being held in Hamilton to discuss an application seeking approval for the modified reassessment of four active ingredients used in 13 pesticides that kill aquatic plant pests.
The Agrichemicals Reassessment Group, made up of the Ministry for Primary Industry, the Department of Conservation, Land Information New Zealand, Mighty River Power and 12 regional or district councils, is seeking removal of controls that prohibit or restrict the application of Metsulfuron-methyl, Haloxyfop-R-methyl, Imazapyr isopropylamine and Triclopyr triethylamine on to or into water.
Niwa principal scientist Paul Champion said there were 16 aquatic pest plant species being targeted for eradication nationally and the 13 currently banned products containing either of the ingredients were acknowledged as "the best way to manage the pest weeds for eradication".
In his evidence, Mr Champion said the chemicals would be used at levels that were likely to be non-toxic to humans and wildlife such as eels and fish.
While the herbicides could damage non-target plants, this was likely to be minimal when compared with effects caused by pest plants without control. "In my opinion the threats posed by the weed species targeted for management using these products are far greater and more enduring than the impacts arising from the use of these chemicals."
But a number of people spoke out against the chemicals including Bob Tait, a mechanical engineer and volunteer for environmental group Friends of the Earth.
Mr Tait said there was insufficient information on environmental and human health effects of applying the range of chemicals above or into water.
He also questioned the consultation, saying there were several interested parties including Watercare and the Auckland District Health Board who were not properly notified.
Poto Davies, of the Ngati Koroki Kahukura Trust, said the Waikato River once overflowed with eels - but not any more.
Her people supported alternative methods of pest control and wanted assurances there would be no non-target effects from the use of the chemicals and that any residual application did not bio-accumulate.
Bill Chisholm, of the South Island Eel Industry Association, said the commercial eel industry had an export value of $5 million and could be adversely affected. He said Metsulfuron-methyl was known to remain persistent in the environment in alkaline waters, which most New Zealand waters are.
"For commercial eel fishermen, the risks relate to the potential complete shutdown of their export markets, if any pesticide residues are found in the flesh ... of exported eels," he said.
DoC senior technical adviser Keith Briden said the agrichemical approach was the most effective.
The EPA will inspect a number of sites around the Waikato today before making its decision, which is due in mid-December.
Who are the Agrichemicals Reassessment Group?
A collection of 12 regional councils, DoC, Land Information NZ, Mighty River Power and Niwa.
What do they want?
The "modified reassessment" under the Hazardous Substances and New Organisms Act 1996 of 13 aquatic pesticides containing a number of active ingredients banned from use in and around waterways.
What are these active ingredients and what do they do?
Metsulfuron-methyl, Haloxyfop-R-methyl, Imazapyr isopropylamine and Triclopyr triethylamine. They are used in pesticides for eradication of plant matter but are prohibited from use in or around water.
What are some of the weeds in question?
Sagittaria, Manchurian wild rice, alligator weed.