Maori have closed access to the upper reaches of the Tongariro River in an effort to minimise the risk of contamination from the invasive algae didymo.

Dead cells of the algae, commonly known as rocksnot, 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.

Biosecurity experts are urgently carrying out more tests for fear that didymo is now in North Island rivers.

Today the Tuwharetoa Maori Trust Board said it was deeply concerned at the discovery of traces of didymo in rivers around the Tongariro National Park, and that the key consideration now must be the future health of the river.

The board said that the peaks of Tongariro and the streams and rivers which flowed from them had always been, and remained to this day, an integral part of Tuwharetoa life.

Trust board secretary Rakeipoho Taiaroa said tribal members would be highly distressed if didymo was confirmed as being in local waterways.

He said the Trust Board was part of the Central North Island Didymo Steering Group comprising Biosecurity NZ, the Department of Conservation and Genesis Power.

"We have nothing but praise for the way the authorities have responded to the initial discovery of dead didymo cells in these waterways," Mr Taiaroa said.

"However, the most important consideration for Tuwharetoa at this stage, is what measures will best protect the future health of the river," he said.

Mr Taiaroa said that the trust board and the Lake Rotoaira Forest Trust had closed access to the upper reaches of the Tongariro River to minimise the risk of further contamination.

"Regardless of whether this scare shows live didymo is in those waterways; Tuwharetoa will be reviewing future access to and from waterways across its lands.

"It is likely that future access to those waterways will be more stringently controlled in the future," he said.

Earlier today Tourism Minister Damien O'Connor said media reports about a possible didymo outbreak in the central North Island could unnecessary alarm tourism operators and visitors.

"It is important to note that the infection has not been confirmed and there are no signs of an algal bloom."

There was no need for people to panic as this could cause unnecessary damage to the image of New Zealand as a fishing destination, he said.

Live cells need to be present before a waterway can be confirmed as infected with didymo.

MAF Biosecurity has reminded anglers and other recreational freshwater users to check, clean and dry their equipment between waterways.

Its biosecurity response manager, David Hayes, said staff from MAF, Department of Conservation and Fish & Game would be at popular spots on the affected rivers to remind river users to check and thoroughly decontaminate any equipment they have been using.

The world fly fishing championship is due to be held on the Tongariro River next year.

Didymo was first discovered in the lower Waiau and Mararoa rivers in South Island in late 2004. Last month it was found in the upper Clarence River in Marlborough and in the Kakapotahi River, south of Ross, in Westland.