Testing has found more dead didymo cells in samples collected at two sites on the Tongariro River.
Biosecurity Minister Jim Anderton says every effort is being made through scientific analysis to establish whether the discovery confirms the presence of didymo in the Central North Island.
Dead cells ofthe invasive algae were found last month in water samples taken from the Tongariro, Whakapapa, Mangatepopo and Whanganui Rivers as part of Genesis Energy's monthly water quality sampling programme.
The finds raised fears that didymo, widespread throughout the South Island, had crossed Cook Strait and was now in North Island rivers.
More samples have been collected from the rivers and the results showed dead cells at two sites on the Tongariro only - at Rangipo and Waipakihi.
Mr Anderton said further testing was conducted both upstream and downstream of these sites.
Two negative results, where no dead cells were present, had been received from the Rangipo site.
Negative results were also received from the upstream and downstream test samples from Waipakihi.
Further DNA sampling had been conducted at the six original sites, the results from these tests are due today.
The DNA analysis method was able to detect didymo at very low levels.
Mr Anderton said Genesis Energy had voluntarily shut down power generation at the Tokaanu power station, as operating the station required the diversion of water between river systems.
"We are working with Genesis Energy to assess the risk of re-initiating power generation on the western and eastern diversions," he said.
"Power generation will resume on the western diversion if DNA tests on this system are negative.
"However, because of a different risk profile, the eastern diversion will be subject to additional testing before decisions about resumption can be made."
The Tuwharetoa Maori Trust Board has said it is restricting road access to the Tongariro River and surrounding area, including the Waiouru Army exercise area.
Mr Anderton said the trust board had explained the situation to the Army, which had agreed not to use the exercise area.
The high number of sites at which dead didymo was found was unusual, he said.
MAF Biosecurity New Zealand was investigating whether this might have been the result of cross-contamination during the initial field sampling.
