A blunder by a Government scientific agency was responsible for traces of an invasive pest found in central North Island rivers.
Niwa, which is responsible for protecting New Zealand's waterways, accidentally introduced dead didymo cells by using contaminated lids on water-sampling containers.
Spreading an unwanted organism can draw up tofive years' imprisonment and/or a fine of up to $100,000.
The discovery of the dead didymo cells in water samples from the world-famous trout fishing rivers in October caused widespread fears of ecological and economic calamity.
The pest is not a health risk to humans but grows rapidly and can choke rivers, destroying fish food.
Until the cells were found in test samples taken at six sites on the Tongariro, Whakapapa, Mangatepopo and Whanganui rivers, the pest was thought to be confined to South Island rivers.
Further investigations found no evidence of live didymo in the North Island rivers.
But an audit was held to establish how the dead cells showed up in surveillance samples.
Niwa chief executive John Morgan said yesterday the answer was that lids on water-sampling containers staff used in river testing had been contaminated with dead didymo cells.
The contamination occurred when the lids were in a South Island laboratory where dead didymo cells had been weighed during scientific study.
When Genesis Energy assigned Niwa to test rivers it used to drive electricity turbines, the contaminated testing vials were brought north.
Biosecurity Minister Jim Anderton said rigid laboratory hygiene meant live didymo cells could not have been transferred to the North island.
Fish and Game chief executive Bryce Johnson said last night: "Everyone makes mistakes and we must learn from the huge consequences of this one.
"It caused an amazing reaction that cost time and worry for heaps of people."
