Dairy farmers are being warned to be careful not to spread pests and diseases moving between farms in the coming week, as the milking season comes to a close.
The Waikato Regional Council said it had confirmed dirty machinery was linked to the spread of velvetleaf in the region last year.
Hundreds of dairy farmers are expected to move cows and machinery around the country's roads as they relocate themselves and their stock in time for the new season. The usual moving day - 1 June - is also known as Gypsy Day.
"The biggest risk is dirty machinery leaving the property and of course their animals as well," council pest plants team leader Darion Embling said.
"The ideal situation is where their machinery is cleaned and ready to go ... It's not only your tractor but your quad bikes, your car and other vehicles that are leaving the property."
It was easy to forget the animals could also transport pests and weeds, he said.
"Stand animals down for about 24 hours before they leave the farm, like quarantine I guess, and feed them on dry matter like hay or silage before they go.
"That means things like yellow bristle grass and even velvetleaf have a chance to go through the animal before they are shifted to the new property... and potentially shift those organisms with them which is the last thing we want."