The rate of didymo invasion in South Island rivers is tipped to explode.
The latest prediction by Waikato University reveals the alga, known as "rock snot", could spread to up to 70% of southern waterways in 15 years.
Didymo was first discovered in Southland in October 2004. Within three years it had spread to the Nelson region.
As of 11 March, the official count for the presence of didymo in the South Island is 132 rivers and six lakes.
At this rate of growth, Waikato University scientists say the South Island will soon be saturated, with didymo likely to infect 1801 rivers by 2024.
While Didymo was previously confined to fast flowing streams in higher elevations, it is now thriving in warmer waters and lake areas. The effects of the projected spread are as yet unclear.
Biodiversity professor Craig Carry says a study on the impact of didymo on fish stocks is due to be completed in two years.
Biosecurity New Zealand says so far the North Island remains clear of didymo.