A Fish & Game environmental scientist says a proposal to discharge significantly more nitrates into the Tukituki River would kill the river if it's approved.
The proposal is part of a plan by Hawke's Bay Regional Council to build a dam to support intensive agriculture in central Hawke's Bay.
A government-appointed board will consider a resource consent application to allow an increase in the amount of nitrogen entering the river by up to 250%.
Ms Jordan said such an increase from farming activity like dairying would cause toxic algal blooms that would kill the river.
She said the application appears to be a test case.
Council spokesperson Ian Maxwell said the council has commissioned studies that show nitrate levels are not important in terms of algae growth in the Tukituki.
He said a plan is in place to manage the river's phosphorus levels.