Nearly 200 fish were found dead in the Clive River on Saturday due to weed build-up.
The dead mullet were discovered by Clive River neighbours and Kohupatiki Marae members at Whakatu on Saturday morning.
Marae member Tom McGuire, who helped set up Operation Patiki in 2008 to clean the river, said the fish died because they swam up the river, flipped on top of the thick weeds and got stuck.
Operation Patiki has been working with Hawke's Bay Regional Council (HBRC) to clean the river by planting native trees on the riverbanks and a council weed boat cut the weeds.
However, because the boat only works in segments the cut weeds travel downstream and clog the waterway, causing this problem, Mr McGuire said.
"When they clear it they need to clear a channel all the way down so the fish can get through."
His daughter, Ann McGuire, said the first thing that needs to be done is to completely remove the weeds.
"Somebody's got to do something pretty quick."
When the weeds are chopped by the boat, the roots are still there so they grow back and the weed remnants float down the river and start to grow again in other places, she said.
"When I was little there was a little bit of weed, I used to come here with my father and fish for flounder and swim.
"Now nobody wants to swim here ... the fish can't even swim here.
"People used to live off the river and enjoy living by the river. We want to revitalise the awa."
The weeds get worse in summer and become thicker due to excessive nutrients in the water, she said.
"It's only going to get worse, it's not a pretty sight."
Marae member Aki Paipper, whose property borders the river, said there is no flow in the river because of the weeds, and the fish on Saturday were trapped and clinging for their lives.
"The water cannot traverse out to sea, it blocks up all the fish.
"The river is rendered inhospitable, this is unacceptable.
"I grew up as a child here, the river yielded so much food you could feed a family."
Nutrients from farms, orchards and other industries going into the river need to be monitored and people need to be responsible for what they put into the river, she said.
HBRC communications advisor Susan Wylie said river staff had been contacted and would be looking into the situation.
Because there had been little rain the weeds would not have been able to flow downstream.
"Usually a good bout of rain will push it through."
If a water sample was collected by Operation Patiki on Saturday, HBRC would be able to test it to ensure there was no other cause for the dead fish, she said.