A toxic discharge has killed more than 500 fish in a New Plymouth stream and those responsible could face a fine of $600,000.
Taranaki Regional Council resource management director Fred McLay said hundreds of fish were found dead in the Mangaone Stream.
"This is a major fish kill. There were too many to pick up," Mr McLay said.
It was not known where the pollution had come from but the council hoped to complete its investigations within 10 days.
This was not the first time the stream, which runs through a New Plymouth industrial area, had been affected by pollution, he said.
"We have had fish kills here in the past but not for about eight years."
Ten businesses held 16 consents to discharge wastewater, stormwater and leachate into the lower Waiwhakaiho and the Mangaone Stream.
The council had a monitoring programme in place at the stream, he said.
It was likely those found responsible for the pollution would face prosecution, he said.
Penalties ranged from $600,000 for companies to $300,000 for individuals, but he was not willing to speculate for this case, he said.
"That's up to the courts to decide."
It was too early to determine what toxins poisoned the fish, but the impact was substantial, he said.
Among the native species killed were shortfin eels, longfin eels, redfin bullies, banded kokopu, freshwater crayfish and shrimp.
A number of dead trout had also been found. There had been no signs of bird life killed, he said.
Monitoring of the stream last year showed low numbers of macroinvertebrates such as insects, worms and snails as a result of discharged contaminants.
Last year regional council monitoring manager Keith Brodie said tributyltin (TBT), a pesticide used in timber preservation at Taranaki Sawmills, was found in the water and sediment of Mangaone Stream.
TBT was no longer being used on site so its origins may go back 10 years.
It also suffered from significant nutrient enrichment from stormwater being discharged at a nearby fertiliser distribution depot owned by Ravensdown Fertiliser Co-operative.
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Chemical pollutants seem like a very plausible explanation, to explain the otherwise unexplainable an inexplicable. However if we see an earthquake/Non-viscous Geoplasma Discharge in the next 24-48hrs, I would go with the discharge being the genesis event triggering the fish kills. We saw this happen in earlier mass die-offs and whale beachings in this area, when there were two, Non-Viscous Geoplasma Discharges. In fact if memory serves me correctly I think the initial whale beaching’s may have preceded the quakes, almost like a warning in the water.