Waikato Regional Council staff are working to control a fresh outbreak of the aggressive plant pest Alligator Weed at Ruapuke Beach, between Raglan and Kawhia.
The South American aquatic weed, which is mostly found in water but can also grow on dry land, is a serious threat to productive farm land because its dense roots block waterways.
Regional Council biosecurity officer Wendy Mead says the weed was spotted by a Department of Conservation officer and was probably spread by contaminated fishing nets.
She says the council is also carrying out intensive control of the weed in the Waikato River delta area and on a few dairy and cropping farms near Hamilton.
Alligator weed is already widespread in Northland and has also been found in Auckland and other parts of Waikato.
It arrived in New Zealand in the 1880s in ships' ballast water.
The weed does not seed but breaks easily into small fragments that can regrow.
It is dispersed by water movement such as floods or tides, by soil movement, and by equipment such as diggers, farm machinery, eel nets and boats.
It has a hollow and reddish-green stem and carries white flowers in summer.