Spread of the invasive algae didymosphenia geminata - commonly known as rock snot - is predicted to explode in South Island rivers.
Waikato University researchers predict the pest will spread to up to 70 percent of southern waterways in 15 years.
It spread the length of the South Island, as far as Nelson, within three years of being first discovered in Southland in October 2004.
A total of 132 rivers and six lakes in the South Island have so far been infested, and the Waikato University scientists say the South Island will soon be saturated, with rock snot in 1801 rivers by 2024.
Part of the problem is an apparent change in the behaviours the algae has shown overseas, where it tended to be confined to fast-flowing streams in higher elevations.
Now it is now thriving in warmer waters and lake areas -- some scientists theorise the change has been made possible by exposure to higher levels of ultra-violet light in New Zealand, or because the strain of the algae which reached here was particularly robust.
Another theory has been that the changed behaviour has been triggered by climate change, with warmer winters and reduced flows favouring growth of the algae and its range.
"Didymo is expanding its environmental range," Waikato University biologist Craig Cary said.
"It's moving into warmer water, and into still water. It used to be pretty much confined to cold streams at higher altitudes."
New genetics research has suggested there may have been two original invasions of didymo from both North America and Canada, he said.
A 2006 report assessed the potential value impact of didymo on New Zealand to be between $58 million and $285m over the seven years 2005 to 2012.
The economic analysis quantified the cost in financial terms of the potential impacts identified by a Ministry of Agriculture and Forestry technical advisory group, taking into account the potential range, rate of spread and impacts on several financial sectors.
Prof Cary said the long-term effect of didymo on freshwater fish was still unknown.
"It fouls the rivers and lakes, and looks awful," he said. "It blocks water intakes and streams. The economic impact has to be considered as well."
The algae can form a thick brown layer that smothers rocks, and submerged plants and forms flowing "rats tails" that can turn white at their ends and look similar to toilet paper.
