Nineteen trees, some genetically modified, have been cut down in an apparent protest against Crown forestry research institute Scion.
Those responsible for the attack dug under the Rotorua institute's perimeter fence and left behind a spade with a sticker saying "GE Free New Zealand".
It is unknown exactly when the attack occurred but the trees were discovered slashed on Monday and police were called.
The felled radiata pine included both genetically modified and unmodified trees being grown as part of a field trial.
Police said last night they were unsure who was responsible for the attack but were conducting forensic tests on items seized at the scene.