NZTA said today the established eucalyptus trees had died in the Kinmont road reserve area, near Mosgiel, and would be felled in coming weeks.
NZTA Coastal Otago Maintenance Contract Manager Nick Rodger said an independent investigation revealed the trees were "intentionally drilled and poisoned".
"The actions of the person or persons who have done this are extremely disappointing," Mr Rodger said.
NZTA was keen to let the Mosgiel and Kinmont Park community know that the trees currently did not pose a safety hazard to users of the Kinmont Reserve or to road users on the adjacent SH1.
The reserve would be closed to the public while the trees are felled.
"The trees were well established and were a key feature of the reserve area, which is managed by the transport agency. They were also being retained in the upcoming landscaping project due to start in the next couple of months.
"While the installation of new plants will start in the next couple of months, preparations are underway to remove unhealthy vegetation and prepare the garden beds.
People can expect to see work being undertaken in the reserve areas around the SH1 / SH87 junction."
