Biosecurity officers have been rushed to a North Canterbury farm where another outbreak of Chilean needle grass has been found.
The grass reduces crop yields and causes animal welfare problems.
It is spread by the movement of machinery, stock, hay and people.
The newly discovered infestation, near Waipara, brings the total number of infestations in the region up to 17.
Canterbury Regional Council biosecurity spokesperson Laurence Smith said officers would establish the extent of the outbreak, remove seed heads and spray plants with herbicide.
Neighbouring properties would be searched over the coming weeks to check for spread, he said.