Paraparaumu residents were irked after they saw thousands of plastic balls floating in the Waikanae River.
The small North Island town is north of Wellington and west of Masterton.
Paraparaumu resident Blair Rosson said he went out to the river to take his children swimming on Saturday about 3pm.
When they get to their usual swimming spot, they saw the river was peppered with bobbing plastic balls of different colours, it has been reported.