Larger areas of the Takaka River are now covered by a carpet of the highly invasive alga didymo, which was discovered in the river last December.
Golden Bay Department of Conservation community relations manager Greg Napp, who examined the known didymo sites on the river this week, said its spread had been "amazing".
"In the shallow reaches near Lyndsey's Bridge, there's now a continuous film of didymo from bank to bank," he said.
At Blue Hole, a popular picnic spot near Upper Takaka, the pest was now well established on the rocks.
"The thickness of the mass is increasing and where it is attached to rocks, it has a real rock snot appearance," Mr Napp said.
When didymo was first found at Blue Hole last Christmas, by a Wellington-based biosecurity officer holidaying in Golden Bay, it was in one or two places only, in coin-sized pieces.
However, its spread down the river had been predicted and there was nothing that could be done to remove it, Mr Napp said.
Didymo is also widely spread further down the river at Kotinga Bridge, where it has formed a carpet about 4m wide on rocks on the shallow side of the river.
Mr Napp said there was less evidence of the algae at Waitapu Bridge, probably because the river was tidal at that stage.
DOC didymo manager for the Nelson-Marlborough region, Kerry Brown, said there was no doubt that in areas where didymo formed a thick layer, it would damage a river's ecology. "It can smother the habitat for invertebrates such as mayflies, and this will have a negative impact on fish and birds higher up the food chain.
"And it will be certainly be no fun for people who want to swim in the river."
As there was no means of controlling didymo, DOC's main aim was to minimise its spread by education and information, he said.
The Nelson-Marlborough region now has three times as many didymo rangers on its waterways as last year, in a bid to stop the pest spreading to other rivers. Six rangers are being employed this year, compared with two last year.
Fears that didymo could spread to the internationally significant Te Waikoropupu Springs prompted DOC to impose a ban on any water contact with the springs nearly two years ago.
A watch on the springs is to be stepped up for summer, with the appointment of a DOC ranger for the site, who is due to start work next month.
Didymo was first found in the Nelson region in the Buller River in September 2004. It has since spread to several other rivers in the Buller catchment and the Motueka River, as well as the Takaka River.
- The Nelson Mail
Do you agree with the city council's decision to put a 30-minute limit on buskers' performances?Related story: (See story)