Dead fish and eels have been found in the Waikato River at Hamilton near the site where a truckload of chemicals burst into flames last night.
Officials are concerned at the possibility up to 44kg of organo-phosphate pesticide could have made its way into the waterway.
Environment Waikato spokesman David Stagg saidit was thought fish and eels had died in a tributary, about 50 metres from the river, where an initial flush of chemicals and firefighting water entered the waterway system.
During an inspection earlier today it became clear some chemicals from the burning truck entered the river system from the presence of a red dye that had been among the truck's cargo, Mr Stagg said.
Dead fish and eels had been found in the river, but it was thought they would have died in the trubutary.
"The reason for that is that the dilution starts to be pretty rapid once it gets into the river, and the majority of fish can move out of the way," he said.
The worst case scenario would be if all 44kg of pesticides on the truck got into the water, but there was no way of knowing whether all, or none, of any particular chemical had got into the waterway system.
While much of the material being carted by the truck, which included formaldehyde, ethanol, paints and corrosive acid, had been recovered from the remains, it was not known what had been recovered and what had not.
It was not expected all the pesticide would have made its way into the river, and what was there would have been diluted, Mr Stagg said.
It would not be at a level which would be fatal to anyone, but it could be at a level which should not be drunk, and downstream users were being warned not to drink the water.
Contaminants from the truck would be making their way downriver in a parcel, and work was being done to try to predict when the parcel would pass different places along the river.
