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.
"We don't understand why anyone would want to do this. In an area that can often look quite barren, it's a welcome sight to see a bit of green alongside this main thoroughfare, and this type of tree is an iconic part of Napier," Stewart said.
This is not the first time trees have been attacked along this stretch of highway. About eight years ago several in virtually the same location were cut down.
Council staff said unfortunately tree poisonings around the city were not unheard of.
For example, a large liquidambar in Jeffares Place, Taradale, had to be removed recently as it had been poisoned.
The council asks for anyone with information about who may have poisoned the pines, or sees any suspicious behaviour related to trees on public land, to get in touch as soon as possible.