KEY POINTS:
Didymo has infested a trophy brown trout fishery on the South Island's West Coast and has also put the Inangahua River at risk.
The Department of Conservation said that didymo, also known as rock snot, had been detected in the Waitahu River, the major tributary of the Inangahua.
The infestation was picked up during routine testing.
The Waitahu is only a few kilometres from Reefton.
The Inangahua has tested clear so far but DOC didymo ranger Gordon Roberts said more testing would be done over the next fortnight.
The Waitahu is very accessible and its headwaters are popular with fishermen and hunters.
West Coast Fish and Game field officer Dean Kelly, of Hokitika, said the Waitahu was a trophy fishery with a big population of good-sized fish. It is also quite a fragile fishery.
Anglers probably unwittingly carried the didymo from the nearby Buller River.
However, the area was also popular with four-wheel-drive enthusiasts and hunters.
Mr Kelly said it was now a matter of waiting to see how well established the didymo would become.
Didymo had been in the Haast River for the past year but was still not visible and it was believed that the high rainfall and sediment helped scour it out. The only place it was blooming was in lake outlets.
However, Mr Kelly said the test result showed that the "clean, check and dry" message was not getting across.
- NZPA
