Several hundred dead fish were discovered a few days ago in the Heathcote River in Christchurch, most likely killed by chemical discharge.
Environment Canterbury is investigating the situation.
Christchurch West Melton Zone Delivery lead Nathan Dougherty says the number is huge:
"More than 300 have been killed."
"Probably related to some sort of contaminants going into the river."
Environment Canterbury is asking for information about any suspicious activity.
"We're looking at all the activity in the area to find out what's going on," Dougherty said.
"We appreciate anyone contacting us with any information regarding anything suspicious or could be related to someone discharging something into the river or the water network."
It is urging residents not to enter the water, and make sure pets stay away too.
