Global research into the noxious algae is limited. Anecdotal evidence from the US indicates they can reduce fish populations by up to 90 per cent in localised areas.
The most vulnerable area in the north is that surrounding Lake Taupo, which includes the Tongariro River.
The algae appear to have invaded that river already, as well as the Whanganui, Mangatepopo and Whakapapa rivers.
However, Biosecurity NZ yesterday pointed out that the samples found were unconfirmed, and the cells tested to date had all been dead.
"There's always the possibility that there's been some contamination of the samples," spokeswoman Judith Hamblyn said.
More testing was being carried out at the sites where the cells were found, and the original samples had undergone retesting.
Scientists gave preliminary results of some tests to a meeting with key stakeholders late yesterday.
Results will not be made public until all testing is complete and the full picture is known, Ms Hamblyn said. This would be within the next few days.
The organisation was still holding out hope that live cells in the rivers were not present, although experts say didymo's spread is inevitable.
Environment Waikato biosecurity manager John Simmons said local authorities had met the Department of Conservation, Biosecurity NZ, and Fish & Game over recent years to prepare for the algae's invasion.
Limiting their spread was the first defence, which could be achieved with public campaigns.
The public was urged to wash and dry all fishing gear, boats, kayaks and any other equipment that entered waterways before and after recreation.
The best treatment option for the areas where most algal damage occurred was to flush the infected waterways with chelated copper, Mr Simmons said. This had been shown to get rid of the pest, but it was needed in vast quantities for larger waterway catchments.
Fish & Game chief executive Bryce Johnson said the outlook was not good.
Evidence of dead cells "certainly indicates that live cells have been present, and there's now a strong probability that those catchments have didymo".
There were "massive ramifications" for the industry, particularly around Taupo and Tongariro, world famous for trout fly fishing.
International visitors spent large amounts of money staying at lodges throughout the Tongariro and Taupo areas, and didymo's presence could turn a number of people away.
"It makes fishing just very unpleasant, time consuming and ineffective," Mr Johnson said.
"The whole aesthetic of New Zealand is at risk - our protective clean-green image. It will change the perceptions of the recreational user."
According to the Biosecurity NZ report, written last year, a high-impact infestation in the North Island is predicted to affect tourism returns to the tune of $64 million.
"Existence values" and loss of native species will fall by values of $40 million and $55 million respectively. New screens and maintenance at water intakes, which include hydro power stations and municipal water supplies, are forecast to cost the economy $38 million.
But the biggest impact will be on recreational values, which include hunting, fishing, boating, and swimming. Losses are estimated at $88 million.
Tourism New Zealand said that it, the Ministry of Tourism, and the Tourism Industry Association were in touch with Biosecurity NZ. Agencies were being provided with ongoing updates about the situation.
Rowing New Zealand spokesman Kevin Strickland said any boats that travelled between the South Island's Lake Ruataniwha and Waikato's Lake Karapiro were always cleaned.
He said every effort should be made to keep the algae at bay.
WHAT IS DIDYMO?
Known also as "rock snot", didymo (didymosphenia geminata) is algae that form dense fibrous mats, with a texture and appearance of dirty brown cotton wool.
They can clog waterways and give rivers a polluted look.
The most likely cause of didymo transfer is through human activity, such as failure to clean and dry boats and fishing equipment.
However, there is a small possibility that animals such as birds could also spread the algae.
