The contaminant that killed hundreds of eels and fish in a stream in Christchurch's South Hagley Park was probably a strong chemical cleaning product.
That's the conclusion of an analysis done by the Canterbury Regional Council after the contamination of the stream was spotted last Saturday evening.
The council's environmental protection manager, Brett Aldridge, says a sample from the stream shows high levels of ammonia.
He says it doesn't require a lot of ammonia to kill fish, and in this case a large quantity went down the stream in a short space of time.
Mr Aldridge says they are checking businesses in the area to see if anyone has any information about the contamination.