Norfolk pines planted alongside State Highway 2 between Marine Parade and Awatoto have been poisoned.
Ten of the pines, about eight years old, were poisoned a few weeks ago.
The Napier City Council said it took some time for the effects to be noticed, but drill holes where the poison was inserted into the trees were clearly visible on some of them.
The trees were planted as an extension of the Norfolk pines further up Marine Parade, some of which are well over 150 years old.
Team leader of Parks, Reserves & Sportsgrounds Debra Stewart said they couldn't understand why someone would do this, especially to these iconic trees.