Auckland Council investigators have found that tests into the death of dozens of eels at Te Arai Stream are inconclusive.
On June 5, residents found the dead eels at the mouth of the stream and were concerned they had been poisoned by discharge from a nearby quarry in mid-May.
Council regulatory compliance manager Steve Pearce says tests carried out at the stream, tributaries and farm drainage channels showed there were excessive nutrients in the water.
He says that over the dry period, the water had become oxygen-deficient and, in parts, highly acidic.
“The drought allowed contaminants, including nutrients from rural land uses, to accumulate and concentrate in the various shallow and intermittent farm drains and tributaries leading into the Te Arai Stream,” Mr Pearce said.
He said Council would continue to work closely with the local dairy industry, the quarry and others in the area to ensure any potential contaminants from those sources were adequately monitored and controlled.
As to whether the drainage of a pond from a nearby quarry could have made an impact, he said it was likely “the rain flushed contaminants from many sources”.