The Department of Conservation says it may be just a matter of time before didymo turns up in North Island waterways.
The invasive weed, which is characterised by thick brown mats, has infected rivers and lakes throughout the South Island.
The weed can be transported on boats, footwear and other equipment that has been in contact with infected water.
DoC spokesperson David Cade says the last water test in December in the Taupo area showed there was no didymo.
However, he says it is continuing to spread in the South Island and there is always a chance it will reach the North Island.
Mr Cade says all river and lake users should clean and dry their gear after use.