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Biosecurity Minister Jim Anderton confirmed last night that there is no evidence of live didymo in Central North Island river samples.
But an investigation is under way to find out how dead didymo cells were found in the samples.
MAF Biosecurity NZ went on high alert late last
month when dead cells of the invasive algae, commonly known as rocksnot, were found in water samples taken from the Tongariro, Whakapapa, Mangatepopo and Whanganui rivers as part of Genesis Energy's monthly water quality sampling programme.
Further testing found more dead didymo cells in samples collected at two sites on the Tongariro River.
Mr Anderton said more samples from all six sites had been tested both by microscope and by DNA analysis.
"They have shown no live didymo."
The finds had raised fears that didymo, widespread throughout the South Island, had crossed Cook Strait and was now in North Island rivers.
"That there were no live cells to be found is certainly good news," Mr Anderton said. "But it is important that we establish how it was possible for the dead didymo cells to have shown up in the surveillance samples."
He said MAF Biosecurity was trying to identify the source of the dead didymo cells and was conducting further investigation of the sampling and analysis procedures to assess whether some field or laboratory contamination had occurred.
"As well, there will be further surveillance in the area, on top of the regular monthly surveillance that is already in place."
Mr Anderton said Genesis Energy had recommenced power generation on the western and eastern diversions of the Tokaanu power station, although the eastern diversion had since been closed because of flooding.
The Tuwharetoa Maori Trust Board, which had restricted road access to the Tongariro River and surrounding area, had now lifted the restrictions.
A Central North Island didymo action group has been in place in the region for 18 months and has a response plan that will be implemented, should live didymo be found in the area.
"Since the dead cells were found, the people involved have put in long hours and those organisations have provided significant resources," Mr Anderton said. "The local community has also been very supportive."
Didymo was first discovered in the lower Waiau and Mararoa rivers in South Island in late 2004.
- NZPA
