Circus elephants are believed to be the indirect cause of a large outbreak of alligator weed near Hamilton.
Waikato Regional Council says the pest plant was discovered over a four-hectare area by a spray contractor in April.
The council says the owner had first noticed the weed sprouting after a load of elephant dung was bought as a fertiliser from a circus that visited Hamilton in 2007.
Pest management committee chair Laurie Burdett says the elephants were fed on hay, which the council believes came from further north where alligator weed is more common.
Ms Burdett says the council has checked past circus sites but there is no other sign of the weed.
She says the weed is fast growing and can take years to eradicate.