The tree, near the intersection of Station Road and Box Hill in Khandallah, was attacked on Monday 31 January at about 5.15pm.
The Council received a call from a concerned member of the public that the tree was under attack by men with axes.
The circumference of the tree trunk has been damaged. The bark and an inner layer of plant tissue that transfers nutrients and water within the tree have been removed.
Officers from the Council visited the property at 5.45pm. No one was home at the time or the following day, Tuesday, when officers returned.
Council officers took immediate remedial measures in an attempt to save the heritage tree.
The tree had been the subject of a resource consent for its removal. The application, lodged on 9 September 2010, was declined on 22 December 2010. The matter is now before the Environment Court.
About 160 trees have heritage protection and are listed on the Council's District Plan. Attacking a protected tree is a criminal act.
The Council's Environment Portfolio Leader, Councillor Helene Ritchie, says she is disappointed at what could only be described as an irresponsible act of wilful damage.
"The Council is doing everything it can to save the tree,'' says Cr Ritchie.
"We will pursue the matter and take whatever action we can to ensure the culprits are caught and dealt with in the appropriate manner.''
The Council's Parks and Gardens Manager, Paul Andrews, says the incident has been reported to police and he urged anyone with information to come forward.
"It's disappointing that this wilful act of vandalism has significantly damaged a heritage tree, at a time when the tree is still subject to an appeal in the Environment Court over its heritage status,'' says Mr Andrews.
Anyone with information that may help the Council's investigation is urged to contact the Council on (04) 499 4444 and ask to speak with the compliance officer.