An urgent appeal has gone out to farmers to check maize crops and silage stacks for signs of a virulent weed.
The weed, velvetleaf, which is known as Butter Print in New Zealand, has been found well established in a maize crop in Waihou, Waikato, growing both in the crop and along fence lines.
There are also reports it may have also been found in Helensville, north of Auckland.
Agresearch senior weed scientist Dr Trevor James says velvetleaf is one of the United States' worst plant pests and can reduce maize crops by up to 35%.
Dr James says the weed, which grows to 3 metres in height, has soft hairs on its leaves and yellow flowers.