Crop farmers are being asked to be wary of black-grass, after the invasive weed was found growing in mid-Canterbury.
Federated Farmers said an outbreak had been discovered on three farms in the region last month.
Its arable group chairman, Guy Wigley, said it was likely the weed had come from a consignment of ryegrass imported in 2007.
He said farmers and the Ministry for Primary Industries had been working to clear the weed, which could be difficult to contain once it spread.
"All the plants that have been found to date have been removed from the properties and destroyed.
"The affected areas have also had herbicide put over them to make doubly sure that there's no risk of those plants showing up again and there will be ongoing monitoring of the identified fields and those in the wider area to make sure there's no more plants."
Mr Wigley said farmers needed to be vigilant in watching out for the grass, as a larger incursion would be "devastating" for the arable industry.
"In the United Kingdom it has started to develop resistance to herbicides that selectively take it out of wheat crops, so it has become quite difficult to control," he said.
"Grass is a very competitive weed in wheat, so you don't want something like that grass to become widely established because it will restrict or reduce your yields in wheat by 40 or more percent if you get a bad infestation, so it's something we don't want establishing here."