The Nelson City Council is worried that trees along city streets could die as a result of a mystery poisoning.
About 30 trees, between 5-30 metres high, in Trafalgar and Bridge Streets are looking sick.
The council believes the trees have been poisoned by metsulfuron, a chemical used to control gorse and broom.
It says its weed control contractor's records show it isn't responsible, and further investigations are underway.
The council says if vandalism is suspected, the police will be called in.