The Environmental Defence Society says it may go to the Environment Court to test the claim by many regional councils that they do not have the power to compel farmers to keep stock out of rivers.
EDS chairman Gary Taylor says the society believes the councils have always had that power under the Resource Management Act but have chosen not to use it.
He says the society will consider this week whether to ask the Environment Court for a declaratory statement on the issue.
The society has been studying reports sent to Northland Regional Council by Millan Ruka of Whangarei who has been patrolling rivers and documenting pollution from farms.
He and Wellington businessman Grant Muir, who's monitoring river pollution in Wairarapa, are proposing a national network of river patrols.