The weakness that led to a giant tree crushing cars in a city park and injuring a woman could not have been detected in standard testing.
An independent arborist's report into the fallen cottonwood at Cornwall Park says the breakdown of the tree's wood quality that led to the fall could not have been detected using standard tree assessment techniques.
Damage to the foot plate and lower root collar of the 30m tree combined with wet soil contributed to the fall.
The tree had been inspected in June last year by arborists as part of an annual audit and examination of more than 7000 trees in the park.
At the time it was assessed as having an annual risk of harm of less than one in a million.
The cottonwood was found to be in good health and showed no instability.
Testing with a sound hammer did not highlight any acoustic anomalies, indicating that decay identified on its surface were localised and superficial.
Park director Michael Ayrton said all mature trees were assessed each year.
"We were deeply shocked by this fall, and well aware how traumatic it would have been for the people affected, and we are very grateful at it did not cause serious physical injury," he said.
The park was commissioning a peer review of tree health processes to see if any changes were needed.
Trees that did not meet the assessment criteria would continue to be removed.
West Auckland woman Dallas Hargreaves, 72, was trapped when the cottonwood unexpectedly crashed to the ground over four parked cars.
She was dragged from her mangled vehicle by off-duty police officer Sergeant Adrian Heffernan and treated for leg injuries in hospital.